Reference Manual on Scientific Evidence: Fourth Edition (2025)
Chapter: The Admissibility of Expert Testimony
The Admissibility of Expert Testimony
LIESA L. RICHTER AND DANIEL J. CAPRA
Liesa L. Richter, J.D., is George Lynn Cross Research Professor, Floyd & Martha Norris Chair in Law, University of Oklahoma College of Law, and Academic Consultant to the Judicial Conference Advisory Committee on Evidence Rules.
Daniel J. Capra, J.D., is Philip Reed Professor of Law, Fordham Law School, and Reporter to the Judicial Conference Advisory Committee on Evidence Rules.
CONTENTS
Federal Rule of Evidence 702: An Overview
The Pre-Rules Frye Standard and Original Rule 702
The Supreme Court’s Daubert Trilogy
The 2000 Amendment to Rule 702
Rule 702(a): The Expert’s Opinion “Will Help” the Trier of Fact
Rule 702(b): The Expert’s Opinion Testimony Is Based on Sufficient Facts or Data
Rule 702(c): The Testimony Is the Product of Reliable Principles and Methods
Rule 702(d): The Witness Has Applied the Principles and Methods Reliably to the Facts of the Case
The 2023 Amendment to Rule 702
Procedures for Resolving Rule 702 Motions and Objections
Daubert Motions and Summary Judgment
Credibility Determinations and Rule 702 Motions
Important Procedural Takeaways
Recurring Issues in the Application of Rule 702
Extrapolation and the “Analytical Gap”
Weight of the Evidence Analysis: Closing the Analytical Gap
Introduction
The trial process operates to resolve disputed facts and claims through the presentation of evidence. The vast majority of evidence presented at trial comes in through the testimony of witnesses. Many of these witnesses are lay witnesses with personal knowledge of the facts and underlying events relevant to the issues in dispute. Often, however, the finder of fact requires assistance from witnesses who possess scientific, technical, or other specialized knowledge. Such witnesses offer expert opinion testimony that, when properly validated, helps jurors and judges resolve complex issues that are outside common knowledge and experience. Federal Rule of Evidence 702 regulates the admissibility of expert opinion testimony. Pursuant to Rule 702, the federal trial judge serves as a gatekeeper for expert opinion testimony, ensuring that only reliable expert testimony is considered by the factfinder.
The enactment, in 1975, of Federal Rule of Evidence 702, the interpretation of that rule by the U.S. Supreme Court in Daubert v. Merrell Dow Pharmaceuticals, Inc.,1 and the further amendments to Rule 702 as recently as 2023, have effected a gradual but significant change in how federal courts treat the admissibility of scientific evidence. Previously, a district court had to decide whether the proffered evidence was deduced from scientific principles generally accepted in the corresponding scientific community. Now, the district court must itself determine whether it is more likely than not that these principles are reliable and have been reliably applied to the facts of the case. This requires the court to become conversant with the science at issue—helped perhaps by this publication. At the same time, courts are cautioned not to prefer one valid scientific theory over another and to admit even differing expert opinions that satisfy the Rule 702 reliability standard. This reference guide examines how courts have tried to grapple with this difficult gatekeeper role.
Federal Rule of Evidence 702: An Overview
When a party objects to the admissibility of expert opinion testimony, Rule 702 requires a trial judge to analyze the basis of the challenge and to make certain findings before admitting the testimony.2 Rule 104(a) of the Federal Rules of Evidence requires trial judges to decide preliminary questions regarding the admissibility of evidence by a preponderance of the evidence. Rule 104(a) applies to
1. 509 U.S. 595 (1993).
2. As with other evidence rules, a trial judge need not make findings under Rule 702 in the absence of an objection to expert opinion testimony. See Fed. R. Evid. 702 advisory committee’s note to 2023 amendment.
the trial judge’s findings under Rule 702, as it does to the findings under most of the evidence rules.3 To admit expert opinion testimony over objection, a trial judge must consider the following aspects of the testimony, if challenged. First, the judge must find the witness qualified to opine on the subject matter at issue. The judge must also find that the witness’s scientific, technical, or other specialized knowledge will help the trier of fact. Before admitting the expert’s opinion, the court must find that the opinion is based on sufficient facts or data. In addition, the court must determine that the witness’s opinion is the product of reliable principles and methods and that the opinion reflects a reliable application of those principles and methods to the facts of the case. And a 2023 amendment to Rule 702 clarifies that the trial judge should determine that the expert witness does not overstate the conclusions that may be drawn from the methodology. The proponent of the testimony has the burden of satisfying the challenged aspects of Rule 702 by a preponderance of the evidence. Although Rule 702 characterizes opinion testimony based on scientific, technical, or other specialized knowledge as “expert” opinion testimony, some federal opinions caution trial courts to refrain from referring to witnesses as “experts” to avoid giving them undue influence with the factfinder.4
History of Rule 702
The Pre-Rules Frye Standard and Original Rule 702
In order to understand the current Rule 702 standard, it is helpful to be familiar with the history of expert opinion testimony in American trials and the standard
3. The Supreme Court, in Bourjaily v. United States, 483 U.S. 171 (1987), held that Rule 104(a) requires preliminary questions regarding the admissibility of evidence to be decided by the trial judge using a preponderance of the evidence standard.
4. See, e.g., United States v. Maya, 966 F.3d 493, 505 (6th Cir. 2020) (“We have cautioned district courts not to declare a witness an expert in front of the jury.”); United States v. Bartley, 855 F.2d 547, 552 (8th Cir. 1988) (noting that “[s]uch an offer and finding by the Court might influence the jury in its evaluation of the expert and the better procedure is to avoid an acknowledgment of the witnesses’ expertise by the Court”). See also American Bar Association Civil Trial Practice Standard 14 (2007) (“The court should not, in the presence of the jury, declare that a witness is qualified as an expert or to render an expert opinion, and counsel should not ask the court to do so” because “use of the term ‘expert’ may appear to a jury to be a kind of judicial imprimatur that favors the witness.”); Fed. R. Evid. 702 advisory committee’s note to 2000 amendment (“[T]here is much to be said for a practice that prohibits the use of the term ‘expert’ by both the parties and the court at trial. Such a practice ‘ensures that trial courts do not inadvertently put their stamp of authority’ on a witness’ opinion, and protects against the jury’s being ‘overwhelmed by the so-called “experts.”’”) (quoting Hon. Charles R. Richey, Proposals to Eliminate the Prejudicial Effect of the Use of the Word “Expert” Under the Federal Rules of Evidence in Criminal and Civil Jury Trials, 154 F.R.D. 537, 559 (1994)).
of admissibility that applied before the Federal Rules. In 1923, in the case of Frye v. United States, the D.C. Circuit Court of Appeals held that polygraph evidence was properly excluded because it had not gained “general acceptance” in the scientific community.5 Thereafter, the Frye “general acceptance” standard was widely adopted by federal and state courts alike to regulate the admissibility of expert testimony.6 When the Federal Rules of Evidence took effect in 1975, the Frye “general acceptance” standard reigned. As originally enacted, Rule 702 provided simply that a qualified witness could testify if scientific, technical, or other specialized knowledge would “assist” the trier of fact to understand the evidence or to determine a fact in issue. It did not reference the Frye standard or give any clue to its continued application under the rules.
The Supreme Court’s Daubert Trilogy
Until 1993, there was considerable controversy over whether Federal Rule of Evidence 702 incorporated the Frye “general acceptance” test. Then, in Daubert v. Merrell Dow Pharmaceuticals, Inc.,7 the Supreme Court unanimously rejected the Frye “general acceptance” standard as the exclusive test for assessing the admissibility of scientific expert testimony under Rule 702.
Daubert was a case in which the plaintiffs claimed that the use of anti-nausea drug Bendectin during pregnancy caused birth defects in their babies. The district court granted summary judgment for the defendant after excluding the plaintiffs’ expert on causation under Frye’s general acceptance standard. Because the expert’s reliance on animal studies, chemical structure analysis, and reanalysis of epidemiological studies had not been generally accepted as methods for determining causation, the trial court excluded the expert’s opinion testimony. On appeal, the Supreme Court scrutinized the language of Rule 702 and found nothing pointing to the continued application of the “general acceptance” standard. The Court further opined that a rigid “general acceptance” standard for the admissibility of scientific evidence would be at odds with the “liberal thrust” of the Federal Rules of Evidence and their otherwise flexible approach to opinion testimony. Therefore, the Court held that Rule 702 did not adopt the Frye “general acceptance” standard as the sole measure of the admissibility of expert opinion testimony.
5. 293 F. 1013 (D.C. Cir. 1923).
6. See Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 585 (1993) (“In the 70 years since its formulation in the Frye case, the ‘general acceptance’ test has been the dominant standard for determining the admissibility of novel scientific evidence at trial.”). For a discussion of the admissibility of scientific evidence under the Frye standard, see Edward Imwinkelried et al., Courtroom Criminal Evidence, Vol. 1 §§ 608–613 (7th ed. 2022).
7. 509 U.S. 579 (1993).
Although it rejected “general acceptance” as the sine qua non of admissibility, the Court found limits on the admissibility of scientific expert opinion testimony implied by the terms “scientific” and “knowledge” in Rule 702. According to Daubert, a trial judge must ensure that scientific evidence is not only relevant, but reliable. Rule 702 requires the trial judge to act as a “gatekeeper” for scientific expert testimony, to ensure that it truly proceeds from “scientific . . . knowledge.” The Court stated that an inference or assertion must be derived from the scientific method in order to qualify as scientific knowledge.
Essentially, the Court held that a trial judge must evaluate proffered scientific expert testimony to ensure that it is more likely than not reliable; concerns about the validity and reliability of expert testimony cannot simply be referred to the jury as questions of weight. The Court made this clear by noting that Rule 104(a) provides the standard for admitting challenged expert testimony. Rule 104(a) had previously been interpreted by the Court as requiring admissibility requirements to be established by a preponderance of the evidence.8 Under Daubert, therefore, the judge must find it more likely than not that the expert’s methods are reliable and that they are reliably applied to the facts at hand.
The move from Frye to Daubert dramatically changed the role of federal district judges. Under Frye, the trial judge could essentially count heads in the relevant scientific community to determine whether a particular methodology was generally accepted. Under Daubert, however, the decision about reliability cannot be completely outsourced to the scientific community; it is the trial judge who must examine the proffered scientific opinion testimony to determine whether it is reliable. Thus, a district judge faces the challenging task of understanding the science behind a proffered expert opinion sufficiently to assess its reliability. Chief Justice Rehnquist, dissenting from the majority’s gatekeeper holding in Daubert, lamented that the majority had imposed on trial judges “either the obligation or the authority to become amateur scientists in order to perform that [gatekeeper] role.”9
To assist judges in this new gatekeeping task, the majority in Daubert set forth a five-factor, nondispositive, nonexclusive, “flexible” test to be employed by the trial court under Rule 702 in determining the “validity” of scientific evidence. These factors—the famous “Daubert factors”—are:
- whether the technique or theory can be or has been tested;
- whether the theory or technique has been subject to peer review and publication;
8. The Court held that questions of admissibility under Rule 104(a) must be determined by a preponderance of the evidence in Bourjaily v. United States, 483 U.S. 171 (1987).
9. 509 U.S. at 601.
- the known or potential rate of error;
- the existence and maintenance of standards and controls; and
- the degree to which the theory or technique has been generally accepted in the scientific community.
The factors described by the Court demand an objective assessment of an expert’s technique or theory. With respect to testing, the Court explained that whether a scientific technique or theory has been or can be tested is ordinarily a key question in assessing its reliability because scientific methodology “is based on generating hypotheses and testing them to see if they can be falsified.” The Court explained that publication is an important consideration because submission to the scrutiny of the scientific community increases the likelihood that substantive flaws in the methodology will be detected. Still, the Court recognized that publication is not an absolute requirement of admissibility given that well-grounded but innovative theories may not have been published. Finally, the Court explained that the general acceptance of a technique remains relevant to, although not dispositive of, admissibility. The Court explained that widespread acceptance can be an important factor in finding a methodology reliable.
The Ninth Circuit applied the gatekeeping standard on remand in the Daubert case, offering additional insights into the newly articulated test.10 The court held that summary judgment was properly granted against the plaintiffs because their experts’ testimony that Bendectin caused limb reduction was inadmissible under Rule 702. In so holding, the Ninth Circuit focused on “whether the experts are proposing to testify about matters growing naturally and directly out of research they have conducted independent of the litigation, or whether they have developed their opinions expressly for purposes of testifying.” According to the court, “a scientist’s normal workplace is the lab or the field, not the courtroom or the lawyer’s office.”11 Thus, when expert opinion testimony is based on research conducted independent of litigation, this “provides important, objective proof that the research comports with the dictates of good science.” The court reasoned that “independent research carries its own indicia of reliability, as it is conducted, so to speak, in the usual course of business and must normally satisfy a variety of standards to attract funding and institutional support.”12
10. Daubert v. Merrell Dow Pharms., Inc., 43 F.3d 1311, 1317–19 (9th Cir. 1995).
11. Id.
12. Id. In casting doubt on research conducted in anticipation of litigation, the Daubert on remand court referred to research on issues of general, as opposed to specific, causation. In a toxic tort case, the plaintiff must show both that the toxic substance causes the harm suffered by the plaintiff (the issue of general causation) and that the toxic substance actually did cause the plaintiff’s harm in the instant case (the issue of specific causation). Daubert on remand expressed concern about research into general causation that is conducted in anticipation of litigation. But an expert cannot be expected to conduct research on specific causation—the cause of a particular plaintiff’s
The Ninth Circuit did not hold that an opinion that is not based on independent research is inadmissible. Instead, the court explained that “the party proffering it must come forward with other objective, verifiable evidence that the testimony is based on scientifically valid principles” if the expert’s testimony is not based on independent research.13 According to the Ninth Circuit, this means that “the experts must explain precisely how they went about reaching their conclusions and point to some objective source—a learned treatise, the policy statement of a professional association, a published article in a reputable scientific journal or the like—to show that they have followed the scientific method, as it is practiced by (at least) a recognized minority of scientists in their field.”14
Applying these principles to the testimony of the plaintiffs’ experts, the Daubert on remand court found that the Daubert standards had not been met. The experts’ research on the question of general causation was prepared solely for purposes of Bendectin litigation. It had not been peer reviewed, nor had it even been deemed worthy of comment by the scientific community. Finally, the experts had not explained their methodology or verified it by reference to objective sources. The court concluded that it had been presented “with only the experts’ qualifications, their conclusions and their assurances of reliability. Under Daubert, that’s not enough.”15
Federal opinions since Daubert have continued to articulate factors that may be important in assessing the admissibility of expert testimony, which are well summarized by the court in In re Marriott Int’l, Inc., Customer Data Sec. Breach Litig.: whether the expert will be testifying about matters that grow “naturally and directly out of research they have conducted independent of litigation, or whether they have developed their opinions expressly for purposes of testifying”; whether “the expert has unjustifiably extrapolated from an accepted premise to an unfounded conclusion”; whether “the expert has adequately accounted for obvious alternative explanations”; whether “the expert is being as careful as he would be in his regular professional work outside his paid litigation consulting”;
injury—outside the realm of litigation. It is only when the plaintiff discovers the injury that research into causation would begin, and it is also at that very point that litigation is anticipated. Indeed, every treating doctor who testifies to the plaintiff’s injuries can be said to conduct her research in anticipation of litigation.
13. See also Wendell v. GlaxoSmithKline LLC, 858 F.3d 1227, 1235 (9th Cir. 2017) (“[T]he district court was wrong to put so much weight on the fact that the experts’ opinions were not developed independently of litigation and had not been published. While independent research into the topic at issue is helpful to establish reliability, its absence does not mean the experts’ methods were unreliable . . . [E]xpert testimony may still be reliable and admissible without peer review and publication. That is especially true when dealing with rare diseases that do not impel published studies.”) (citations omitted).
14. See Daubert, 43 F.3d 1311, 1318 at n.9.
15. Id. at 1319.
and whether “the field of expertise claimed by the expert is known to reach reliable results for the type of opinion the expert would give.”16
The Daubert opinion sent some mixed messages and left some unanswered questions, however. It sent mixed messages by criticizing the rigidity of the Frye general acceptance standard as being too “austere” and by encouraging a “flexible” inquiry, while at the same time directing trial judges to ensure that an expert’s opinion was properly derived from the “scientific method.” It clearly announced the trial judge’s obligation to find reliability by a preponderance of the evidence under Rule 104(a), but then noted that “[v]igorous cross-examination, presentation of contrary evidence, and careful instruction on the burden of proof are the traditional and appropriate means of attacking shaky but admissible evidence.”17 Of course, the trial methods referred to by the Court are the proper methods for attacking testimony—once it has been found admissible. But this language caused some to assume, mistakenly, that “shaky” expert opinion testimony should be admitted, with issues of reliability to be resolved by the jury as ones of weight.
Daubert also left open questions. The opinion very clearly described the standard for admitting “scientific” expert testimony, leaving open the questions of which testimony falls within this category, as well as the standard applicable to nonscientific expert opinion testimony. Finally, because the Supreme Court remanded the case to the lower courts, it did not apply the reliability standard announced in Daubert to a concrete set of facts, leaving the lower courts to apply the new standard in the first instance.
Some of these open questions were addressed by the Supreme Court soon after. The Court offered insight into the proper application of the Daubert reliability standard in General Electric Co. v. Joiner.18 In Joiner, the plaintiff proffered experts to testify that his exposure to polychlorinated biphenyls (PCBs) promoted his development of small cell lung cancer. The district court excluded the plaintiff’s experts as unreliable under Daubert and granted summary judgment for the defendants. On appeal, the Eleventh Circuit reversed, stating that “[b]ecause the Federal Rules of Evidence governing expert testimony display a preference for admissibility, we apply a particularly stringent standard of review to the trial judge’s exclusion of expert testimony.”19 Under this stringent standard of review, the Eleventh Circuit reversed the exclusion of the plaintiff’s experts.
On appeal, the Supreme Court first held that the abuse of discretion standard of review applies to a district court’s rulings admitting or excluding expert testimony under Rule 702 and that the Eleventh Circuit had erred in applying a
16. In re Marriott Int’l, Inc., Customer Data Sec. Breach Litig., 602 F. Supp. 3d 767, 774 (D. Md. 2022).
17. 509 U.S. at 596.
18. 522 U.S. 136, 146 (1997).
19. Joiner v. Gen. Elec. Co., 78 F.3d 524 (11th Cir. 1996).
more rigid standard.20 Thus, appellate review of a trial court’s Daubert ruling proceeds in two steps. The appellate court reviews de novo whether a trial court correctly applied the Daubert/Rule 702 framework and reviews for abuse of discretion the trial court’s ruling admitting or excluding expert testimony.21 The Supreme Court in Joiner found no abuse of discretion in the trial court’s exclusion of the plaintiff’s experts as unreliable, emphasizing that a trial judge must ensure that an expert’s application of her principles and methods to the facts of the case is also reliable. The experts relied on several studies, but the Court found that these studies did not support the experts’ conclusions as to causation. In one study, infant mice were exposed to PCBs and contracted a different kind of cancer than that suffered by the plaintiff. Moreover, the studies could not be replicated in adult mice or in any other species. The experts’ reliance on four epidemiological studies was likewise flawed. Two of the studies found no statistically significant connection between PCBs and cancer; one study did not mention PCBs; and the fourth study, which found a statistically significant connection, involved subjects who were exposed to a variety of other carcinogens. The Court concluded that there was an “analytical gap” between the experts’ conclusion on causation and the studies on which they relied for their conclusion. The experts made no attempt to bridge this analytical gap by relying on any other objective or verifiable standards. The Joiner Court concluded that the expert testimony was merely “ipse dixit”—it is so because I am an expert and I say so. According to the Court, such testimony fails to fulfill the Daubert requirement that an expert’s conclusions be based on an objective and verifiable methodology.
Justice Stevens dissented from this portion of the opinion, arguing that the majority had examined each study relied upon by the plaintiff’s experts in a piecemeal fashion and concluded that the experts’ opinions on causation were unreliable because no one study supported causation. Justice Stevens argued that a “weight of the evidence” approach to scientific reasoning, in which the experts relied on all of the studies taken together, as well as interviews of the plaintiff and an examination of his medical records to conclude that his exposure to PCBs promoted his development of lung cancer, was appropriate and acceptable under Daubert.22
Daubert stated that trial courts should focus “on principles and methodology, not on the conclusions that they generate.” But the Joiner Court clarified that a trial court should not let an opinion pass through the gate of admissibility when there is a clear disconnect between the stated methods and the conclusion reached by the expert. The Joiner Court explained that “conclusions and
20. 522 U.S. at 143.
21. Kirk v. Clark Equip. Co., 991 F.3d 865, 872 (7th Cir. 2021).
22. Id. For a discussion of the “weight of the evidence” methodology, see section titled “Weight of the Evidence Analysis: Closing the Analytical Gap” below.
methodology are not entirely distinct from one another” because if an expert uses a reliable method and comes to an outlier conclusion, the gatekeeper can fairly assume that there is some disconnect in the expert’s application of that methodology. According to the Court, an expert must not only be using a reliable methodology (like reviewing studies, as in Joiner); she must also be reliably applying that methodology to the facts at issue. As one court has stated, trial judges “may evaluate the data offered to support an expert’s bottom-line opinions to determine if that data provides adequate support to mark the expert’s testimony as reliable.”23
Joiner did not authorize trial courts “to determine which of several competing scientific theories has the best provenance.”24 Further, “Daubert does not require that a party who proffers expert testimony carry the burden of proving to the judge that the expert’s assessment of the situation is actually correct.”25 Qualified scientific experts applying reliable methodologies might still have reasonable differences of opinion. The proponent of the evidence must show only that the expert’s conclusion has been arrived at in a scientifically sound and methodologically reliable fashion.
Another question left open by Daubert was whether its standards apply to expert testimony that does not purport to be scientifically based. In Kumho Tire Co. v. Carmichael,26 the Court held that the Daubert reliability standard applies to nonscientific expert opinion testimony. Kumho involved a tire failure expert who would have testified that a tire exploded because it was defective. His mode of analysis was to investigate whether the failure might have been caused for any reason other than defect. He employed his own four-factor test to rule out other causes such as underinflation. He found that some of his self-selected factors indicated improper inflation of the tire. Still, the expert proposed to testify that improper inflation was not the cause of the tire failure. The trial judge excluded the expert’s testimony for, among other reasons, failure to satisfy the Daubert standards. The judge found that the expert’s methodology was subjective: it had not been peer reviewed; there was no indication of the rate of error; and there was no general acceptance of the expert’s four-factor test for determining alternative causation.
On appeal, the Supreme Court reasoned that Rule 702 requires experts to testify on the basis of “knowledge,” and that in Daubert, “the Court specified that it is the rule’s word ‘knowledge,’ not the modifying words like ‘scientific’, that ‘establishes a standard of evidentiary reliability.’” The Court also noted that “it would prove difficult, if not impossible, for judges to administer evidentiary
23. Ruiz-Troche v. Pepsi Cola of P.R. Bottling Co., 161 F.3d 77, 81 (1st Cir. 1998).
24. Id.
25. See Fed. R. Evid. 702 advisory committee’s note to 2000 amendment (“When a trial court, applying this amendment, rules that an expert’s testimony is reliable, this does not necessarily mean that contradictory testimony is unreliable.”).
26. 526 U.S. 137, 147–48 (1999).
rules under which a gatekeeping obligation depended upon a distinction between ‘scientific’ knowledge and ‘technical’ or ‘other specialized’ knowledge. There is no clear line that divides the one from the others.”27 The Court therefore found that the Rule 702 reliability standard applies to all expert opinion testimony—whether supported by scientific, technical, or other specialized knowledge.
The Court then examined whether a trial judge could assess nonscientific expert testimony using the specific factors that Daubert had listed as relevant to assessing the reliability of scientific expert testimony. The Court held that any or all of these factors might be used to evaluate nonscientific expert testimony, but found that the trial judge must have considerable leeway in deciding how to determine whether particular expert testimony is reliable:
Daubert’s list of specific factors neither necessarily nor exclusively applies to all experts or in every case. Rather, the law grants a district court the same broad latitude when it decides how to determine reliability as it enjoys in respect to its ultimate reliability determination.28
The Court emphasized that the factors that a trial judge uses to determine reliability must be dependent upon the particular area of expertise that is being evaluated. The Court explained as follows:
[W]e can neither rule out, nor rule in, for all cases and for all time the applicability of the factors mentioned in Daubert, nor can we now do so for subsets of cases categorized by category of expert or by kind of evidence. Too much depends upon the particular circumstances of the particular case at issue.
At the same time . . . some of Daubert’s questions can help to evaluate the reliability even of experience-based testimony. In certain cases, it will be appropriate for the trial judge to ask, for example, how often an engineering expert’s experience-based methodology has produced erroneous results, or whether such a method is generally accepted in the relevant engineering community. Likewise, it will at times be useful to ask even of a witness whose expertise is based purely on experience, say, a perfume tester able to distinguish among 140 odors at a sniff, whether his preparation is of a kind that others in the field would recognize as acceptable.29
The Kumho Court provided a bottom-line standard for employing the gatekeeping requirement in evaluating nonscientific expert testimony:
The objective of that requirement . . . is to make certain that an expert, whether basing testimony upon professional studies or personal experience, employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.30
27. Id. at 148.
28. Id. at 142.
29. 526 U.S. at 151.
30. Id. at 152.
Turning to the tire failure expert’s proffered testimony in Kumho, the Court found no abuse of discretion in either the trial judge’s use of specific Daubert factors or in the ultimate decision to exclude the testimony as insufficiently reliable. The expert’s assumption—that alternative causes could be ruled out by looking at a number of specific factors identified solely by the expert—was essentially subjective and unsupported by any other tire failure expert. Moreover, the expert discounted evidence that under his own test indicated that the tire had been improperly inflated. The Court concluded as follows:
Indeed, no one has argued that Carlson himself, were he still working for Michelin, would have concluded in a report to his employer that a similar tire was similarly defective on grounds identical to those upon which he rested his conclusion here. Of course, Carlson himself claimed that his method was accurate, but, as we pointed out in Joiner, “nothing in either Daubert or the Federal Rules of Evidence requires a district court to admit opinion evidence that is connected to existing data only by the ipse dixit of the expert.”31
Thus, the expert was properly rejected because he could not show that he used the same intellectual rigor in reaching his conclusion as would be expected of him in his professional life outside the courtroom.
Amendments to Rule 702
Rule 702 has been amended since its original enactment to clarify the trial judge’s gatekeeping role announced in Daubert. In 2000, the rule was amended to incorporate the teachings of the Supreme Court’s Daubert trilogy. In 2011, Rule 702 was amended as part of the restyling project that was designed to make the rules “more easily understood and to make style and terminology consistent throughout the rules.” In 2023, Rule 702 was amended again to emphasize the trial judge’s obligation to find its requirements satisfied by a preponderance of the evidence and to highlight the problem of expert overstatement.32 Because the amendments to Rule 702 have codified principles governing the admissibility of expert opinion testimony found in the caselaw, pre-amendment cases may remain instructive in applying Rule 702, as amended. Judges and litigants should exercise caution in relying on pre-amendment cases, however, because the 2023 amendment was designed to correct misapplications of the rule in some federal cases.33
31. Id. at 157.
32. The amendment took effect on December 1, 2023.
33. See Fed. R. Evid. 702 advisory committee’s note to 2023 amendment (“. . . many courts have held that the critical questions of the sufficiency of an expert’s basis, and the application of the expert’s methodology, are questions of weight and not admissibility. These rulings are an incorrect application of Rules 702 and 104(a).”).
The 2000 Amendment to Rule 702
In 2000, Rule 702 was amended for two purposes: 1) to codify, structure, and expand upon the Supreme Court trilogy of Daubert, Joiner, and Kumho and its extensive progeny; and 2) to provide an extensive Committee Note to assist trial courts in exercising the gatekeeping function allocated to them under Daubert.34 The amendment retained the preexisting standards for qualifying an expert witness and for ensuring that expert opinion testimony “will help” the jury (which were part of the original rule and were placed in a new subdivision (a)). The amendment added subsections (b)–(d) to the structure of the rule, setting forth three reliability-based standards that must be met before a challenged expert’s testimony can be admitted.
Rule 702: A Qualified Expert
Amended Rule 702 retained the requirement that an expert be qualified by “knowledge, skill, experience, training, or education.” This flexible qualification standard allows courts to qualify expert witnesses on a variety of grounds and in many disciplines. For example, in United States v. Protho, a panel of the Seventh Circuit upheld the district court’s decision to admit expert opinion testimony about the make, model, and year of a kidnapper’s vehicle from surveillance videos despite the defendant’s objection that the witness lacked the requisite expertise. The appellate court found that the witness’s “position, engineering education, and nearly three decades at Ford make him abundantly qualified to opine on the appearance and identity of Ford’s products.”35 In Cedar Lodge Plantation, L.L.C. v. CSHV Fairway View I, L.L.C., the district court found the plaintiff’s environmental expert unqualified to offer reliable expert testimony because his experience was related to the resolution of hazardous waste matters for commercial and industrial facilities, rather than sewage systems for apartment complexes or multi-family residential communities like the one at issue in the case.36 But a panel of the Fifth Circuit, in an unpublished opinion, found that the expert had “extensive experience in analysis and evaluation of environmental
34. See In re Marriott Int’l, Inc., Customer Data Sec. Breach Litig., No. 19-MD-2879, 2022 WL 1323139, at *4 (D. Md. May 3, 2022) (“The advisory note to the 2000 amendments to Rule 702 (‘2000 Advisory Note’) is essential reading for judges and lawyers who undertake a Daubert analysis.”).
35. United States v. Protho, 41 F.4th 812, 823 (7th Cir. 2022). See also United States v. Smith, 919 F.3d 825, 835 (4th Cir. 2019) (“twelve-year veteran of the FBI with substantial experience in drug and gang investigations (conducting controlled buys, interviewing drug dealers and gang members, using gang members as confidential sources, listening to thousands of recorded conversations, and so forth)” qualified as an expert in drug and gang terminology).
36. 753 F. App’x 191, 195–96 (5th Cir. 2018).
contaminants, the area in which he was offered as an expert, including experience working on sewage systems for residential neighborhood communities.” The court held that the witness’s lack of specialization in sewage facilities for multi-family residential units did not render him unqualified.
Rule 702(a): The Expert’s Opinion “Will Help” the Trier of Fact
Rule 702 also requires that expert opinion testimony “will help” the trier of fact “to understand the evidence or to determine a fact in issue.” “[T]he ‘help’ requirement [from Rule 702] is satisfied where the expert testimony advances the trier of fact’s understanding to any degree.”37 The original Advisory Committee’s note to Rule 702 quoted a law review article explaining the helpfulness requirement as follows:
There is no more certain test for determining when experts may be used than the common sense inquiry whether the untrained layman would be qualified to determine intelligently and to the best possible degree the particular issue without enlightenment from those having a specialized understanding of the subject involved in the dispute.38
According to the original Advisory Committee’s note, unhelpful opinions are “superfluous and a waste of time.” In Daubert, the Supreme Court also characterized this requirement as one of relevance and “fit” between the opinion testimony and the dispute at issue.39
United States v. King offers an example of a court evaluating an expert’s helpfulness. In King, the defendant in a pill mill case objected to the testimony of a medical expert regarding standards of practice in the field of pain management, guidelines for prescribing medications, and “red flags” that indicated potential painkiller abuse.40 The Eighth Circuit affirmed the admission of the testimony even though the expert could not definitively state that any particular prescription was illegitimate. The court found that the expert’s opinion on the general operation of the clinic “advanced the trier of fact’s understanding of the clinical practices at [the clinics] and how they differed from ordinary medical facilities.” With regard to experts relying on the scientific method, it is safe to assume that
37. See, e.g., United States ex rel. Morsell v. Symantec Corp., 2020 U.S. Dist. LEXIS 54847, *12 (D.D.C. 2020) (quoting 29 Charles Alan Wright & Arthur R. Miller, Federal Practice & Procedure § 6264.1 (2015)).
38. See Fed. R. Evid. 702 advisory committee’s note to 1973 amendment (quoting Ladd, Expert Testimony, 5 Vand. L. Rev. 414, 418 (1952)).
39. 509 U.S. 579, 591 (1993).
40. 898 F.3d 797, 805–06 (8th Cir. 2018).
if the testimony is relevant, it will be helpful to factfinders who are unschooled in science.
Rule 702(b): The Expert’s Opinion Testimony Is Based on Sufficient Facts or Data
This subsection requires an expert to have a sufficient foundation for opinion testimony. It imposes a quantitative requirement, ensuring that an expert has enough basis supporting the opinion to which she testifies.41 If an expert has engaged in insufficient research, or has ignored obvious factors, the opinion must be excluded under this prong of the test.42 Put colloquially, an expert must do her homework before offering opinion testimony. A requirement that an expert have enough data to support an opinion flows naturally from the Supreme Court’s demand for “reliable” opinion testimony in Daubert. While this foundation requirement was “well developed in the case law and in the experience of trial lawyers and judges” prior to the 2000 amendment, it was not expressly grounded in one of the federal rules.43
A good example of a faulty foundation is found in Stephens v. Union Pacific Railroad.44 The plaintiff in that case suffered from mesothelioma. The plaintiff’s father worked for Union Pacific when the plaintiff was a young child. The plaintiff claimed that he was exposed to asbestos fibers as a child from his father’s work clothes. In an effort to defeat a summary judgment motion by the defendant, the plaintiff proffered two experts who concluded that the plaintiff’s exposure to asbestos fibers from his father’s work clothes was a substantial factor in causing his mesothelioma. A key premise underlying both experts’ opinions was that the plaintiff was exposed to significant levels of asbestos as a result of his father’s work. In affirming summary judgment for the defendant, the Ninth Circuit emphasized that neither expert had any information about the degree to which the plaintiff’s father was exposed to asbestos fibers in his job, or about the plaintiff’s own level of exposure, if any. Without that information, the court explained that the experts had insufficient basis to conclude that the plaintiff had
41. See Fed. R. Evid. 702 advisory committee’s note to 2000 amendment (“Subpart (1) of Rule 702 calls for a quantitative rather than qualitative analysis.”).
42. See, e.g., In re Incretin-Based Therapies Prod. Liab. Litig., No. 21-55342, 2022 WL 898595, at *1 (9th Cir. Mar. 28, 2022) (“district court properly relied on the uncontested fact that Dr. Gale did not independently review studies that had been published between 2015 and Dr. Gale’s final 2019 report, all of which found no causal relationship between liraglutide use and the development of pancreatic cancer”).
43. Elcock v. Kmart Corp., 233 F.3d 734, 756, n.13 (3d Cir. 2000).
44. 935 F.3d 852 (9th Cir. 2019).
been exposed to asbestos with any regularity. Therefore, the experts’ conclusions were based on insufficient facts or data.45
As explained by the Advisory Committee’s note to the 2000 amendment, “[t]here has been some confusion over the relationship between Rules 702 and 703” and their respective roles in regulating the basis for an expert opinion. Rule 702(b) provides a quantitative requirement. It demands “sufficient” facts or data supporting an expert’s opinion, without which the opinion would not be reliable. Rule 703 addresses the type or quality of the facts or data that may be relied on by an expert. It releases expert witnesses from the standard requirement of personal knowledge, allowing them to base opinions on “facts or data in the case that the expert has been made aware of or personally observed.” It then addresses the situation in which an expert relies on facts or data that would be inadmissible in evidence under the Rules to form the basis of an opinion. Rule 703 provides that experts may rely on such inadmissible basis information so long as “experts in the particular field would reasonably rely on those kinds of facts or data in forming an opinion on the subject.”46 For example, in Nicholson v. Biomet, Inc., a biomedical engineer offered an opinion based, in part, on medical literature and reports that constituted inadmissible hearsay.47 Because biomedical engineers “unquestionably rely on such data from medical experts when designing medical devices compatible with the human body,” the appellate court found no error in allowing the testimony.48 Importantly, Rule 703 prohibits the proponent of the expert from disclosing inadmissible basis information to the factfinder unless the “probative value in helping the jury evaluate the opinion substantially outweighs” its prejudicial effect. In sum, Rule 702 requires a sufficient quantity of facts or data supporting an expert opinion, while Rule 703 regulates the type of data upon which an expert may rely, as well as the disclosure of inadmissible basis to the jury by the proponent.
45. Id.; see also Taylor v. Bristol Myers Squibb Co., 93 F.4th 339, 348 (6th Cir. 2024) (expert who relies on one study and ignores contrary studies, for no justifiable reason, is relying on insufficient facts or data; court notes that the 2023 amendment to Rule 702 was intended to correct court decisions holding that the sufficiency of an expert’s basis was a question of weight and not admissibility); Wasson v. Peabody Coal Co., 542 F.3d 1172 (7th Cir. 2008) (expert’s testimony that lessee’s coal price for calculating royalties was too low based on only a single customer; as such it was not based on sufficient facts or data and was properly excluded under Rule 702(b)).
46. See, e.g., Nicholson v. Biomet, Inc., 46 F.4th 757 (8th Cir. 2022) (“Biomedical engineers, such as Truman herself, unquestionably rely on such data from medical experts when designing medical devices compatible with the human body. Thus, Truman correctly used medical reports and literature as contemplated by Rule 703 to support her opinion as a biomedical engineer on the design of the M2a Magnum.”).
47. Id.
48. Id.
Rule 702(c): The Testimony Is the Product of Reliable Principles and Methods
This subsection was added to Rule 702 by the 2000 amendment to reflect the heart of the Daubert opinion in rule text. As the Court in Daubert held, the trial judge must act as a gatekeeper and must find that the principles and methods utilized by an expert to formulate an opinion are reliable.
United States v. Gissantaner offers an analysis of the reliable methodology requirement.49 The defendant in that case was charged with the unlawful possession of a firearm by a convicted felon after a witness complained that the defendant was handling a gun and a gun was found in his roommate’s drawer. The handle of the gun contained DNA that emanated from multiple sources and the prosecution sought to present testimony based on STRmix, a probabilistic genotyping software program. The testimony would have shown that the software program produced a 49 million to 1 likelihood ratio for the defendant’s DNA, offering “very strong support” for the conclusion that the defendant was a contributor to the DNA sample on the handle of the gun. The district court excluded the STRmix testimony under Daubert, finding it insufficiently reliable as a methodology.
On appeal, the Sixth Circuit conducted a careful analysis of the reliability of STRmix as a method of identifying contributing sources to multisource DNA samples, and reversed. First, the court noted that the STRmix methodology was capable of being tested and had, in fact, been tested using lab-created multisource DNA samples to evaluate the ability of the software to include and exclude sources. Next, the court explained that STRmix has been subjected to peer review because it has been the subject of over 50 peer-reviewed scientific articles. The court noted that two of three studies performed on STRmix were conducted by individuals who were not connected to the development of the software. The court examined the error rate for the STRmix program, finding that STRmix accurately excluded noncontributors to a DNA source 99.1% of the time and produced extremely low likelihood ratios for the 1% of sources that it failed to exclude. The court also noted that a national association of forensic laboratories had developed standards regarding the use of probabilistic genotyping software that were used to minimize errors associated with STRmix and other similar software programs. Finally, the court examined the general acceptance of STRmix as a method for determining contributors to a multisource DNA sample. The court found that STRmix is the “market leader” in probabilistic genotype software that is utilized by the FBI and numerous government laboratories in the U.S. and abroad. After carefully examining each of
49. 990 F.3d 457, 468 (6th Cir. 2021).
the factors outlined in Daubert, the court found that STRmix was a reliable methodology under Rule 702(c).50
It will often be the case that experts in a particular field employ different methods. Daubert and Rule 702(c) do not authorize a trial judge to choose one competing methodology over another if both have been shown to be reliable. The question is whether the methodology employed by the expert is reliable; dispute in the field is a relevant consideration, but it is not dispositive. For example, in Ruiz-Troche v. Pepsi Cola of P.R. Bottling Co.,51 a wrongful death action arising from a traffic accident, the defendant proffered an expert pharmacologist who concluded that the decedent had “snorted” at least 200 milligrams of cocaine thirty to sixty minutes prior to the accident. To reach this conclusion on dosage, the expert relied on a mathematical formula said to be supported by several published studies utilizing a “half-life technique.” The trial court excluded this testimony on the ground that other pharmacologists used a different technique for determining dosage. But the appellate court found an abuse of discretion. The court recognized that the scientific literature relied on by the expert “does not make an open-and-shut case” because the studies indicate that the half-life of cocaine varies among individuals due to many factors. But the court stated that the trial judge “set the bar too high” in excluding the expert’s testimony on the basis of these possible variables. The court noted that “Daubert neither requires nor empowers trial courts to determine which of several competing scientific theories has the best provenance. It demands only that the expert’s conclusion has been arrived at in a scientifically sound and methodologically reliable fashion.” In this case, the expert’s opinion was “premised on an accepted technique” and “embodied a methodology that has significant support in the relevant universe of scientific literature.”52
Rule 702(d): The Witness Has Applied the Principles and Methods Reliably to the Facts of the Case
This subsection was added to Rule 702 in 2000 to require a trial court “to scrutinize not only the principles and methods used by the expert, but also whether
50. Id.; see also United States v. Morgan, 45 F.4th 192 (D.C. Cir. 2022) (expert’s cell tower location opinion supported by “drive testing” was based on reliable principles and methods where “drive testing technology has been relied upon, tested and reviewed for decades in the multibillion dollar wireless communications industry”).
51. 161 F.3d 77, 85 (1st Cir. 1998).
52. Id.; see also Adams v. LabCorp of Am., 760 F.3d 1322 (11th Cir. 2014) (reversing district court’s exclusion of plaintiffs’ expert regarding the breach of a cytotechnologist’s standard of care; expert testimony was admissible notwithstanding nonblinded review where expert used a well-established classification system to identify precancerous cells).
those principles and methods have been properly applied to the facts of the case.”53 This requirement was implicit in Daubert’s recognition of a gatekeeping role for the trial judge and explicit in Joiner. Although Daubert stated that trial courts should focus “on principles and methodology, not on the conclusions that they generate,” the Joiner Court explained that “conclusions and methodology are not entirely distinct from one another.” Thus, Rule 702 was amended in 2000 to make this reliable application requirement explicit in rule text. The Advisory Committee’s note to the amendment explained:
Under the amendment, as under Daubert, when an expert purports to apply principles and methods in accordance with professional standards, and yet reaches a conclusion that other experts in the field would not reach, the trial court may fairly suspect that the principles and methods have not been faithfully applied.
For example, in McGill v. BP Exploration & Production, Inc., the plaintiff brought suit for illnesses he allegedly suffered as a result of his exposure to oil, dispersants, and decontaminants while participating in the cleanup of the Deepwater Horizon oil spill.54 The district court excluded the plaintiff’s causation expert, concluding that the expert lacked critical knowledge regarding the level of the alleged contaminants harmful to humans and the level of the plaintiff’s exposure. The district court also excluded the opinion because the expert assumed that the plaintiff’s illnesses were caused by exposure because of the temporal proximity between his injuries and the exposure.
A panel of the Fifth Circuit upheld the exclusion of the plaintiff’s causation expert in an unpublished opinion. The expert relied on studies showing respiratory harm from exposure to the contaminants at issue. Still, the studies did not support the expert’s conclusion that the plaintiff’s illnesses resulted from his exposure because they did not show that the contaminants caused the illnesses from which the plaintiff suffered or the levels of exposure necessary to cause harm. The court explained:
[T]here is a notable analytical gap between the facts he relies on and the conclusions he reaches. Dr. Stogner’s deposition fails to address other potential causes of McGill’s illness and the method by which he rules them out. Dr. Stogner fails to analyze the conditions of exposure McGill may have experienced. . . . Dr. Stogner was unable to answer questions regarding how much time McGill spent scooping up oil, how, where, or in what quantity Corexit was used, how exposure levels would change once substances were diluted in seawater, or how McGill’s protective equipment would affect exposure.
53. Fed. R. Evid. 702 advisory committee’s note to 2000 amendment.
54. 830 F. App’x 430, 433 (5th Cir. 2020).
Thus, the expert did not reliably apply his principles and methods to the facts of the plaintiff’s case.55
Importantly, as explained in the Advisory Committee’s note to the 2000 amendment, the trial court’s obligation to determine that an expert has reliably applied her methodology does not authorize a judge to determine the correctness of an expert’s conclusion. As the Ninth Circuit explained in Elosu v. Middlefork Ranch Inc.: “Although a district court may screen an expert opinion for reliability, and may reject testimony that is wholly speculative, it may not weigh the expert’s conclusions or assume a factfinding role.”56 In that case, plaintiffs seeking recovery for a fire that burned their cabin to the ground proffered a report by a fire investigator. The expert opined that the fire was caused by an open-flame pilot light that ignited combustible vapors from an oil stain that had been applied to a wooden deck the day before the fire. Although the parties stipulated to the expert’s qualifications and to the reliability of his methodology, the district court excluded his causation opinion for disagreement with its “ultimate conclusions.” The trial court found that the substance of the opinion was “speculative, uncertain, and contradicted by multiple eyewitness accounts.” The Ninth Circuit reversed the exclusion of the expert’s testimony, finding that the trial court “disregarded much of the expert’s scientific analysis, weighed the evidence on record, and demanded corroboration—factfinding steps that exceed the court’s gatekeeping role.” The court emphasized that “Rule 702 does not license a court to engage in freeform factfinding, to select between competing versions of the evidence, or to determine the veracity of the expert’s conclusions at the admissibility stage.”57
55. Id. In McKiver v. Murphy-Brown, LLC, the defendant objected to an expert’s testimony that a DNA marker of hog feces could be found on the homes neighboring defendant’s farm. 980 F.3d 937, 959 (4th Cir. 2020). The defendant contested the application of the DNA methodology in the case and argued that the expert’s qualifications and reliance on the DNA marker could not support an opinion about odor at the neighboring property. The appellate court affirmed admission of the testimony, finding that “Pig2bac, upon which the report was based, has been used globally to demonstrate the traceability of swine fecal wastes and was applied in this case using protocols for sampling and analysis by a team of four Ph.D. holders, each with experience with field work, animal operations, and environmental health and engineering.” The court further noted that the expert’s DNA methodology was properly applied to support his opinion about tracing DNA compounds and that he did not offer an opinion on odor. See also United States v. Morgan, 45 F.4th 192 (D.C. Cir. 2022) (agent reliably applied “drive testing” methodology to approximate cell phone location when he confirmed that there were no changes in cell tower networks since the incident at issue before conducting the testing and subjected his draft analysis to peer review to detect and correct any application errors).
56. 26 F.4th 1017, 1020 (9th Cir. 2022). See also Kennedy v. Collagen Corp., 161 F.3d 1226, 1230 (9th Cir. 1998) (reversing exclusion of causation expert where studies upon which expert relied offered support for his conclusion that collagen injections caused lupus and stating “[j]udges in jury trials should not exclude [expert] testimony simply because they disagree with the conclusions of the expert”).
57. Elosu, 26 F.4th at 1020.
The 2023 Amendment to Rule 702
Rule 702 was amended in 2023 to address two concerns.58 First, Daubert clearly held that Rule 104(a) governs a trial court’s admissibility decision under Rule 702. Rule 104(a) provides that the court must decide any preliminary question about whether evidence is admissible. In Bourjaily v. United States, the Supreme Court held that Rule 104(a) requires the proponent to demonstrate to the court that admissibility requirements are met by a preponderance of the evidence.59 The Advisory Committee’s note to the 2000 amendment to Rule 702 reiterated that this preponderance standard of proof applies to the trial judge’s findings with respect to the admissibility of expert opinion testimony.60 Despite this, many federal opinions have included incorrect characterizations of the applicable standard, stating that questions of the sufficiency of an expert’s basis and the reliability of an expert’s application are questions of weight for the jury. These cases are highly problematic because they largely abdicate the trial judge’s gatekeeping role on the important questions of the sufficiency of an expert’s basis and the reliability of an expert’s application of a methodology to the facts of the case.61
Rule 702 was amended in 2023 to correct these misstatements and to clarify the applicable preponderance standard of proof.62 The amendment expressly states that the proponent of expert opinion testimony must “demonstrate[] to the court” that the admissibility requirements of Rule 702 are “more likely than not” satisfied.63
The Advisory Committee’s note to the 2023 amendment was careful to point out that some challenges to expert testimony will raise matters of weight rather than admissibility even under the proper Rule 104(a) standard:
For example, if the court finds it more likely than not that an expert has a sufficient basis to support an opinion, the fact that the expert has not read every single study that exists will raise a question of weight and not admissibility. But
58. The amendment became effective December 1, 2023.
59. 483 U.S. 171 (1987).
60. See Fed. R. Evid. 702 advisory committee’s note to 2000 amendment (“the proponent has the burden of establishing that the pertinent admissibility requirements are met by a preponderance of the evidence”).
61. Language incorrectly stating that the sufficiency of facts or data and reliable application are generally questions of weight and not admissibility can be found in Loudermill v. Dow Chemical Co., 863 F.2d 566 (8th Cir. 1988); Viterbo v. Dow Chemical Co., 826 F.2d 420 (5th Cir. 1987); and Smith v. Ford Motor Co., 215 F.3d 713 (7th Cir. 2000).
62. See Fed. R. Evid. 702 advisory committee’s note to 2023 amendment (“the rule has been amended to clarify and emphasize that expert testimony may not be admitted unless the proponent demonstrates to the court that it is more likely than not that the proffered testimony meets the admissibility requirements set forth in the rule”).
63. The Advisory Committee’s note emphasizes that “there is no intent to raise any negative inference regarding the applicability of the Rule 104(a) standard of proof for other rules.” Fed. R. Evid. 702 advisory committee’s note to 2023 amendment.
this does not mean, as certain courts have held, that arguments about the sufficiency of an expert’s basis always go to weight and not admissibility. Rather it means that once the court has found it more likely than not that the admissibility requirement has been met, any attack by the opponent will go only to the weight of the evidence.
Second, Rule 702 was amended in 2023 to address a common application problem in which an expert relies on reliable methods and principles to generate an opinion but overstates the conclusion that can reliably be drawn from the methodology. Rule 702(d) was amended to emphasize that each expert opinion must “stay within the bounds of what can be concluded from a reliable application of the expert’s basis and methodology.”64 The Advisory Committee note to the amendment explains that, while the problem of expert overstatement can occur in any context, it is “especially pertinent to the testimony of forensic experts in both criminal and civil cases.” For example, in United States v. Machado-Erazo, a cell phone location expert testified to the precise location of the defendant’s phone.65 The D.C. Circuit held that it was error to permit this testimony where current cell phone location technology can locate a phone only within a general area.66 In contrast, the Seventh Circuit in United States v. Protho held that a forensic fiber analyst’s testimony reliably applied forensic fiber analysis methods because the analyst “acknowledged that her results could not definitively identify fibers as coming from the same source” and “disclosed that fiber analysis can ‘never’ associate ‘a single item to the exclusion of all others’ and that consistency alone ‘is not a means of positive identification.’”67 The 2023 amendment to Rule
64. Id.
65. 901 F.3d 326 (D.C. Cir. 2018).
66. Id.; see also United States v. Hill, 818 F.3d 289, 295 (7th Cir. 2016) (declaring that cell site analysis expert testimony should include a “disclaimer” regarding accuracy; the expert should not “overpromise on the technique’s precision or fail to account for its flaws”; cell site testimony was admissible where the agent testified that the defendant’s phone records were “consistent” with him being at or near relevant locations at relevant times, but clarified that he could not state whether a phone was “absolutely at a specific address”).
67. 41 F.4th 812, 821 (7th Cir. 2022). The reliability of forensic fiber comparison evidence has been subject to serious criticism. See Nat’l Research Council, Strengthening Forensic Science in the United States: A Path Forward 162–63 (2009) (highlighting the lack of research determining the error rate of certain fiber-comparison methods). A concurring opinion in Protho found that the forensic fiber analysis should have been excluded because of the government’s failure to show that it relied on reliable principles and methods under Rule 702(c). Still, the concurring opinion noted that the fiber expert attempted to refrain from overpromising on the capabilities of her analysis. Protho, 41 F.4th at 835 (“To Baloga’s credit, she did not explicitly state that any two fibers ‘matched’ or claim to predict the likelihood that they came from a common source with any statistical precision. But she implied as much by repeatedly insisting during her testimony that one would not expect two random fibers to have all the common characteristics that she identified.”). For a case involving expert overstatement outside the forensic context, see United States v. Valencia-Lopez, 971 F.3d 891 (9th Cir. 2020). The defendant in Valencia-Lopez contended that he was coerced to carry drugs for a cartel. The trial court allowed a law enforcement expert to testify that the
702(d) seeks to rein in expert overstatement by directing trial judges to determine that an expert’s proffered conclusions “reflect[] a reliable application of the principles and methods to the facts of the case.”
Procedures for Resolving Rule 702 Motions and Objections
Trial courts enjoy significant discretion in fashioning procedures for determining the admissibility of expert opinion testimony in a given case. As the Supreme Court explained in Kumho Tire:
The trial court must have the same kind of latitude in deciding how to test an expert’s reliability, and to decide whether or when special briefing or other proceedings are needed to investigate reliability, as it enjoys when it decides whether or not that expert’s relevant testimony is reliable. Our opinion in Joiner makes clear that a court of appeals is to apply an abuse-of-discretion standard when it “review[s] a trial court’s decision to admit or exclude expert testimony.” 522 U.S. at 138–139, 118 S. Ct. 512. That standard applies as much to the trial court’s decisions about how to determine reliability as to its ultimate conclusion. Otherwise, the trial judge would lack the discretionary authority needed both to avoid unnecessary “reliability” proceedings in ordinary cases where the reliability of an expert’s methods is properly taken for granted, and to require appropriate proceedings in the less usual or more complex cases where cause for questioning the expert’s reliability arises.68
Accordingly, trial judges may decide Daubert motions at trial based on voir dire of a proffered expert witness or before trial, based on briefing and supporting documentation or after lengthy Daubert hearings involving testimony from proffered expert witnesses. Whether to hold a Daubert hearing is within the discretion of the court.69
Although the trial court’s gatekeeper review is often conducted through a Daubert hearing, a hearing is not required when the evidentiary record pertinent to the expert opinions is already well developed. For example, in Miller v. Baker Implement Co.,70 the court found that the trial judge did not abuse discretion in excluding the plaintiff’s expert testimony without a Daubert hearing. It noted
likelihood of a cartel using a coerced courier was “almost nil.” The Ninth Circuit found that this opinion was erroneously admitted under Rule 702 because there was no showing of how the expert’s experience could lead to a conclusion of such certainty.
68. 526 U.S. 137, 152 (1999) (emphasis in original).
69. See Fed. R. Evid. 702 advisory committee’s note to 2000 amendment (noting that the rule “makes no attempt to set forth procedural requirements for exercising the trial court’s gatekeeping function over expert testimony”).
70. 439 F.3d 407, 412 (8th Cir. 2006).
that the plaintiff submitted affidavits, a detailed explanation of the proposed expert testimony, and a legal memorandum addressing the expert evidence issues. The court noted that while Daubert hearings “may be necessary in some cases, the basic requirement under the law is that parties have an opportunity to be heard before the district court makes its decisions.”71
Daubert Motions and Summary Judgment
In civil cases, objections to expert testimony under Rule 702 are often made in the context of a motion for summary judgment. Rule 56 of the Federal Rules of Civil Procedure requires that material cited to support or dispute a fact in the summary judgment context can be “presented in a form that would be admissible in evidence.” Therefore, a court must make admissibility determinations to resolve a motion for summary judgment. Defendants often file a Daubert motion in conjunction with a motion for summary judgment. They argue that plaintiff’s proffered expert opinion testimony fails to satisfy Rule 702 and that, without a necessary expert opinion to support the plaintiff on a crucial issue, the defendant is entitled to summary judgment. As the court explained in Cortes-Irizarry v. Corporation Insular,72 “[i]f proffered expert testimony fails to cross Daubert’s threshold for admissibility, a district court may exclude that evidence from consideration when passing upon a motion for summary judgment.” The court in that case cautioned against trying to “gauge the reliability of expert proof on a truncated record,” however. Although the trial court enjoys significant discretion in fashioning proper procedures for evaluating Daubert motions, courts should ensure that the proponent of the challenged expert testimony has an adequate opportunity to develop a record supporting admissibility at the summary judgment stage. A trial court may even wish to require a full presentation of the expert testimony, and rebuttal, to facilitate the admissibility determination. Courts have displayed considerable ingenuity in devising mechanisms that permit Daubert rulings to be made in conjunction with motions for summary judgment.73
71. Id.; see also, e.g., McKiver v. Murphy-Brown, LLC, 980 F.3d 937 (4th Cir. 2020) (noting that the trial court “was entitled to rely on the parties’ materials without requiring further submissions or a Daubert hearing” and that the trial court’s ruling from the bench “reflected that it considered the parties’ arguments and briefing”).
72. 111 F.3d 184, 188 (1st Cir. 1997).
73. See, e.g., Padillas v. Stork-Gamco, Inc., 186 F.3d 412, 418 (3d Cir. 1999) (“An in limine hearing will obviously not be required whenever a Daubert objection is raised to a proffer of expert evidence. Whether to hold one rests in the sound discretion of the district court. But when the ruling on admissibility turns on factual issues, as it does here, at least in the summary judgment context, failure to hold such a hearing may be an abuse of discretion.”); In re Paoli R.R. Yard PCB Litig., 35 F.3d 717, 736, 739 (3d Cir. 1994) (discussing the use of in limine hearings); Claar v.
Credibility Determinations and Rule 702 Motions
As discussed above, trial judges must find the requirements of Rule 702 satisfied by a preponderance of the evidence before passing expert opinion testimony on to the jury. Questions sometimes arise about the judge’s ability to make credibility determinations in performing this function. At first glance, it would seem impermissible for a trial judge to make a credibility assessment because it would encroach on the jury’s fundamental role. But there are some special credibility determinations that may be necessary in evaluating Rule 702’s reliability requirements.
The court considered the complex relationship between expert credibility and reliability in Elcock v. Kmart Corp.74 The trial judge in Elcock held a Daubert hearing and determined that one of the plaintiff’s experts did not pass the reliability threshold. The judge relied, in part, on the fact that the expert had engaged in criminal acts involving fraud, although this fraudulent activity was not in any way related to the expert’s professional life. The Third Circuit found the trial court’s reliance on the expert’s prior bad acts and consideration of the expert’s general credibility to be error. Still, the court made clear that a district judge is required to make some credibility choices as the Rule 702 gatekeeper:
Consider a case in which an expert witness, during a Daubert hearing, claims to have looked at the key data that informed his proffered methodology, while the opponent offers testimony suggesting that the expert had not in fact conducted such an examination. Under such a scenario, a district court would necessarily have to address and resolve the credibility issue raised by the conflicting testimony in order to arrive at a conclusion regarding the reliability of the methodology at issue.75
While the court concluded that some credibility determinations would have to be made at a Daubert hearing, it emphasized that those determinations should be limited to testimony about how the expert reached her opinion, as opposed to witness credibility more generally. Thus, a distinction must be drawn between credibility evidence that bears directly on the expert’s methods and application, and evidence on credibility that is more general.76
Burlington N. R.R., 29 F.3d 499, 502–05 (9th Cir. 1994) (discussing the district court’s technique of ordering experts to submit serial affidavits explaining the reasoning and methodology underlying their conclusions).
74. 233 F.3d 734, 750–51 (3d Cir. 2000).
75. Id.
76. See Adams v. LabCorp of Am., 760 F.3d 1322 (11th Cir. 2014) (issues of potential bias as a result of nonblinded review and expert’s philosophical bent were matters of weight for the jury); see also Edward J. Imwinkelreid, Trial Judges—Gatekeepers or Usurpers? Can the Trial Judge Critically
If the attack on credibility has nothing to do with the expert’s methods, but only with the expert’s general character for truthfulness or bias, the issue of credibility should be left to the jury—the opponent can bring impeachment evidence before the jury by way of cross-examination as with any witness. As applied to the facts of Elcock, the credibility evidence should not have been used by the trial court because it related to acts of dishonesty and fraud completely outside the expert’s work in the particular case.77
Important Procedural Takeaways
Whether or not the trial court holds a Daubert hearing, it is important to keep in mind three procedural points regarding the gatekeeper function:
- The trial court must find that the proponent has or has not shown by a preponderance of the evidence that the expert’s testimony satisfies the requirements of Rule 702. The trial court cannot simply refuse to decide whether the expert relied on sufficient facts and data to support her opinion, employed reliable methods and principles, and applied those methods reliably to the facts of the case at hand. Leaving these matters “to the jury” constitutes an abdication of the trial court’s gatekeeper role under Rule 104(a). For example, the trial court in McClain v. Metabolife Int’l, Inc.78 held a Daubert hearing regarding two witnesses who were providing opinions on pharmacology and chemistry. After the hearing, the trial court essentially washed its hands of the admissibility determination and sent both witnesses to the jury. The trial court explained that it did not “pretend to know enough to formulate a logical basis for a preclusionary order that would necessarily find, as a matter of law, that these witnesses cannot express to a jury the opinions they articulated to the court.”
Assess the Admissibility of Expert Testimony Without Invading the Jury’s Province to Evaluate the Credibility and Weight of the Testimony, 84 Marq. L. Rev. 1, 39 (2000) (stating that an expert’s prior dishonesty or misconduct should not be considered by the court when the acts are wholly unrelated to the expert’s use of a particular methodology, but that a court should take such dishonesty or misconduct into account when the nexus between the acts and the expert’s methodology is more direct, e.g., when the prior dishonest acts involve fraud committed in connection with the earlier phases of a research project that serves as the foundation for the expert’s proffered opinion).
77. See also, Cruz-Vazquez v. Mennonite Gen. Hosp. Inc., 613 F.3d 54 (1st Cir. 2010) (error to exclude expert because he was biased in favor of plaintiffs in medical cases and was generally affiliated with plaintiffs’ lawyers; those considerations are for the jury in assessing the weight of the expert’s testimony).
78. 401 F.3d 1233, 1239 (11th Cir. 2005).
-
The court of appeals found that the trial court had made an error of law by having “essentially abdicated its gatekeeping role. Although the trial court conducted a Daubert hearing, and both witnesses were subject to a thorough and extensive examination, the court ultimately disavowed its ability to handle the Daubert issues. This abdication was in itself an abuse of discretion.”79
- The trial court must make findings on the record and set forth its reasoning on the record as well. Expressly stating that the court has or has not found the Rule 702 requirements satisfied by a preponderance of the evidence can demonstrate that the proper standard has been applied. Without such findings, there is no way for an appellate court to determine whether the gatekeeping function was properly exercised.80
- The gatekeeping function has been exercised flexibly by some courts in the context of bench trials. While Rule 702 certainly applies in bench trials, some courts have found that the gatekeeping function can be applied more flexibly because a trial judge serving as factfinder can be trusted to give dubious expert testimony the weight it deserves.81
Recurring Issues in the Application of Rule 702
There are several recurring issues bearing on the reliability of a scientific expert’s testimony that courts have confronted in deciding motions under Rule 702. None of the issues discussed below is necessarily dispositive, but each has been considered in the cases.
79. Id.
80. See, e.g., Certain Underwriters at Lloyd’s, London v. Axon Pressure Prods. Inc., 951 F.3d 248 (5th Cir. 2020) (abuse of discretion to exclude expert testimony by a plenary order that offered no reasons for the ruling); United States v. Yeley-Davis, 632 F.3d 673 (10th Cir. 2011) (it was error to permit an agent to testify about how cell phone towers work without making findings on the record that the witness was qualified and his opinions were reliable); Smith v. Jenkins, 732 F.3d 51 (1st Cir. 2013) (remand required given the absence of any findings on the record regarding the reliability of the expert’s opinion).
81. See, e.g., Doe v. Hamilton Cnty. Bd. of Educ., 392 F.3d 840, 852 (6th Cir. 2004) (noting that the gatekeeper function was “designed to protect juries and is largely irrelevant in the context of a bench trial”); United States v. Brown, 415 F.3d 1257, 1268 (11th Cir. 2005) (Admissibility standards of Rule 702 “are more relaxed in a bench trial situation, where the judge is serving as factfinder and we are not concerned about dumping a barrage of questionable scientific evidence on a jury. . . . There is less need for the gatekeeper to keep the gate when the gatekeeper is keeping the gate only for himself.”). But see UGI Sunbury v. Permanent Easement for 1.7575 Acres, 949 F.3d 825 (3d Cir. 2020) (finding error in admitting speculative expert testimony, and expressing skepticism about the cases stating that the gatekeeping function is relaxed in bench trials).
Extrapolation and the “Analytical Gap”
In forming their opinions, some experts start from an accepted scientific fact or premise but draw a conclusion that is not necessarily supported by the accepted premise. These experts have sometimes relied on logical analysis to go from premise to conclusion. But others have simply made an unsupported leap. If the process from premise to conclusion is not itself consistent with the scientific method or other well-accepted principles, then courts tend to exclude the testimony as based on unreliable and improper extrapolation.
For example, improper extrapolation occurs when an expert concludes, without supporting research, that a substance that causes one harm also causes a different harm. In Lust v. Merrell Dow Pharmaceuticals, Inc.,82 the question was whether the plaintiff’s birth defect, hemifacial microsomia, was caused by his mother’s use of the drug Clomid. The expert based his conclusion to that effect on epidemiological studies that showed a link between Clomid and other types of birth defects. He concluded that because Clomid is capable of causing other birth defects, it also caused the plaintiff’s hemifacial microsomia. The court explained that the expert “could have convinced the district court that his methodology was nevertheless scientific if he had explained how he went about reaching his conclusions and had pointed to an objective source demonstrating that his method and premises were generally accepted by or espoused by a recognized minority of teratologists.” The court affirmed the trial court’s exclusion of the expert’s opinion, however, because he failed to do so, finding that the expert could not simply rely on epidemiological studies regarding the connection between Clomid and other birth defects to reach a conclusion about the plaintiff’s distinct birth defect. The court found that the doctor’s testimony “was influenced by litigation-driven financial incentive,” and the doctor’s premise—that a positive association between a drug and some birth defects indicates an association with other birth defects—was not recognized by even a minority of scientists.83
Red flags are also raised when a scientific expert purports to testify on the basis of a methodology that she transfers from one arena to a completely different area of inquiry—at least if there is no independent research supporting the
82. 89 F.3d 594, 597 (9th Cir. 1996).
83. See also Mitchell v. Gencorp., 165 F.3d 778, 782 (10th Cir. 1999) (experts could not testify that the plaintiff’s exposure to chemicals similar to benzene would affect the body in the same way as benzene; without “scientific data supporting their conclusions that chemicals similar to benzene cause the same problems as benzene, the analytical gap in the experts’ testimony is simply too wide for the opinions to establish causation”); In re Nexium Esomeprazole, 662 F. App’x 528, 530 (9th Cir. 2016) (opinion of orthopedic surgeon that drug Nexium could cause reduced bone mineral density and related fractures properly excluded where he “did not adequately explain how he inferred a causal relationship from epidemiological studies that did not come to such a conclusion themselves”).
transfer. Thus, in Braun v. Lorillard Inc.,84 the plaintiff sought to prove that the decedent’s mesothelioma was caused by smoking cigarettes with a filter made with crocidolite asbestos. The decedent’s lung tissue was tested for asbestos fibers, using the standard methodologies for the testing of human tissue of “bleach digestion” and “low temperature plasma ashing.” No crocidolite fibers were found. The plaintiffs then retained an expert whose specialty was testing for asbestos in building materials to conduct tests on the decedent’s lung tissue. This expert was unaware of the methodologies ordinarily employed in testing human tissue. He used the same test that he used on building materials to test the decedent’s lung tissue, known as “high temperature ashing,” and found crocidolite fibers in the tissue. The expert stated that high temperature ashing was as usable on tissue as on bricks—though he had never conducted such a test on tissue before this litigation. He also admitted that the high temperature could alter the chemistry of the sample, in which case it would be impossible to tell whether asbestos fibers were crocidolite or some other kind. But he asserted that his method was far more likely to produce a false negative than a false positive.
The Seventh Circuit, in an opinion by Judge Posner, held that the expert’s testimony was properly excluded as unscientific under Daubert. It made the following points:
A judge or jury is not equipped to evaluate scientific innovations. If, therefore, an expert proposes to depart from the generally accepted methodology of his field and embark upon a sea of scientific uncertainty, the court may appropriately insist that he ground his departure in demonstrable and scrupulous adherence to the scientist’s creed of meticulous and objective inquiry. To forsake the accepted methods without even inquiring why they are the accepted methods—in this case, why specialists in testing human tissues for asbestos fibers have never used the familiar high temperature ashing method—and without even knowing what the accepted methods are, strikes us . . . as irresponsible.85
Judge Posner provided a bottom line: Daubert seeks to curtail “the hiring of reputable scientists, impressively credentialed, to testify for a fee to propositions that they have not arrived at through the methods that they use when they are doing their regular professional work rather than being paid to give an opinion helpful to one side in a lawsuit.”86
All this is not to say that learning from one subject matter can never be used to form a conclusion as to a different subject matter, or that premises established in one context are completely irrelevant in another. Extrapolation by an expert is permissible when justified by scientific support or reasoning. An example of permissible extrapolation by a scientific expert arose in Kennedy v. Collagen
84. 84 F.3d 230, 234 (7th Cir. 1996).
85. Id. at 235.
86. Id.
Corp.87 The plaintiff claimed that her use of the facial product Zyderm caused her to contract lupus, an autoimmune disorder. The plaintiff’s expert rheumatologist testified to causation on the basis of peer-reviewed articles, clinical trials, product studies, an investigation conducted by the Texas Department of Health, a physical examination of the plaintiff, laboratory tests, and the temporal proximity between the plaintiff’s use of Zyderm and the onset of her autoimmune disorder. None of the studies upon which the expert relied indicated that Zyderm caused lupus; but they did establish a connection between Zyderm and autoimmune disorders that were similar to lupus. Ordinarily, an analytical gap would arise if the expert were to conclude that Zyderm causes lupus simply because it causes disorders that are similar to lupus. However, the expert filled the analytical gap by relying on the finding, established in both peer-reviewed publications and clinical studies, that Zyderm induces the body to produce the same autoimmune antibodies that are the hallmark of lupus. The expert was not relying solely on causation of other autoimmune diseases, but also on the fact that the manner of causation was the same for lupus and for other autoimmune diseases that were linked to Zyderm. The court concluded that “Dr. Spindler set forth the steps he took in arriving at his conclusion in his deposition. Dr. Spindler’s analogical reasoning was based on objective, verifiable evidence and scientific methodology of the kind traditionally used by rheumatologists. That is precisely what Daubert requires.”88
Weight of the Evidence Analysis: Closing the Analytical Gap
In Joiner, the Supreme Court concluded that there was an “analytical gap” between the experts’ conclusion on causation and the studies upon which they relied. As discussed above, Justice Stevens dissented, arguing that the majority had examined each study relied upon by the experts in a piecemeal fashion and had concluded that the experts’ opinions on causation were unreliable because no one study supported causation. Justice Stevens argued that a “weight of the evidence” approach to scientific reasoning, in which the experts relied upon all of the studies taken together, as well as interviews of the plaintiff and an examination of his medical records to conclude that his exposure to PCBs promoted his development of lung cancer, was appropriate and acceptable under Daubert.89 The majority did not address this argument directly, essentially finding insufficient support for the experts’ conclusion in the studies and data relied upon in that case.
87. 161 F.3d 1226, 1230 (9th Cir. 1998).
88. Id.
89. Gen. Elec. Co. v. Joiner, 522 U.S. 136, 152–54 (1997).
A “weight of the evidence” process of scientific reasoning has been found sufficient to support an expert’s conclusion on general causation under Rule 702 since the Joiner decision. Weight of the evidence methodology allows a scientific expert to reason to a conclusion even though no single report or study exists that independently supports that conclusion. As the First Circuit described it in Milward v. Acuity Specialty Products Group, Inc.,90 the weight of the evidence approach to making causal connections “is a mode of logical reasoning often described as ‘inference to the best explanation’ in which the conclusion is not guaranteed by the premises.”91
The plaintiff in Milward brought negligence claims against chemical companies alleging that the plaintiff’s rare type of leukemia, acute promyelocytic leukemia (APL), was caused by his routine workplace exposure to benzene-containing products. The district court excluded the plaintiff’s general causation expert, finding that his proffered testimony that benzene exposure can cause APL lacked sufficient demonstrated scientific reliability. On appeal, a panel of the First Circuit reversed, finding that the plaintiff’s expert opinion on general causation satisfied Rule 702. The court noted that the plaintiff’s general causation expert relied on a “weight of the evidence” approach in concluding that benzene was capable of causing APL.
The court explained that this approach involves an inference to the best explanation that can be thought of as involving six general steps:
The scientist must (1) identify an association between an exposure and a disease, (2) consider a range of plausible explanations for the association, (3) rank the rival explanations according to their plausibility, (4) seek additional evidence to separate the more plausible from the less plausible explanations, (5) consider all of the relevant available evidence, and (6) integrate the evidence using professional judgment to come to a conclusion about the best explanation.92
The court acknowledged that this method of reasoning depends upon the use of “scientific judgment” but explained: “[t]he fact that the role of judgment in the weight of the evidence approach is more readily apparent than it is in other methodologies does not mean that the approach is any less scientific.”93 The court noted that the weight of the evidence approach is commonly used by scientists, and opined that “[n]o serious argument can be made that the weight of the evidence approach is inherently unreliable” under Rule 702.
The court found that the question to be resolved by a trial court is whether the approach has been properly applied in a particular case. The Milward court found that the district court had erred in excluding the expert’s opinion
90. 639 F.3d 11, 17–19 (1st Cir. 2011).
91. Id.
92. Id.
93. Id.
testimony on general causation where the expert had derived his opinion “from the accumulation of multiple scientifically acceptable inferences from different bodies of evidence” and clearly employed the “same level of intellectual rigor” that he did in his academic work. The Milward court found that the district court erred in treating “the separate evidentiary components of Dr. Smith’s analysis atomistically”:
In Dr. Smith’s weight of the evidence approach, no body of evidence was itself treated as justifying an inference of causation. Rather, each body of evidence was treated as grounds for the subsidiary conclusion that it would, if combined with other evidence, support a causal inference. The district court erred in reasoning that because no one line of evidence supported a reliable inference of causation, an inference of causation based on the totality of the evidence was unreliable. The hallmark of the weight of the evidence approach is reasoning to the best explanation for all of the available evidence.94
The First Circuit explained that the weight of the evidence approach may be particularly appropriate in cases like the plaintiff’s because “the rarity of APL and difficulties of data collection in the United States make it very difficult to perform an epidemiological study of the causes of APL that would yield statistically significant results.”95 Thus, the court held that the trial court abused its discretion by concluding that the expert’s inference of causation based on the totality of the evidence was unreliable “because no one line of evidence supported a reliable inference of causation.”96
94. Id. (citations omitted).
95. Id.
96. See also In re Abilify (Aripiprazole) Prod. Liab. Litig., 299 F. Supp. 3d 1291, 1311 (N.D. Fla. 2018) (“This ‘weight of the evidence’ approach to analyzing causation can be considered reliable, provided the expert considers all available evidence carefully and explains how the relative weight of the various pieces of evidence led to his conclusion.”); Waite v. AII Acquisition Corp., 194 F. Supp. 3d 1298, 1313 (S.D. Fla. 2016) (“This weight-of-the-evidence approach to causation requires consideration of all available scientific evidence, including epidemiology, toxicology (animal studies), cellular studies (in vitro), and molecular biology and has been validated by the courts.”). But see In re Zoloft (Sertraline Hydrochloride) Prod. Liab. Litig., 858 F.3d 787, 796–97 (3d Cir. 2017) (“Here, we accept that the Bradford Hill and weight of the evidence analyses are generally reliable. We also assume that the ‘techniques’ used to implement the analysis (here, meta-analysis, trend analysis, and reanalysis) are themselves reliable. However, we find that Dr. Jewell did not 1) reliably apply the ‘techniques’ to the body of evidence or 2) adequately explain how this analysis supports specified Bradford Hill criteria. Because ‘any step that renders the analysis unreliable under the Daubert factors renders the expert’s testimony inadmissible,’ this is sufficient to show that the District Court did not abuse its discretion in excluding Dr. Jewell’s testimony.”) (citations omitted); In re Nexium Esomeprazole, 662 F. App’x 528, 530 (9th Cir. 2016) (“At best, Dr. Bal analyzed three of the nine Bradford Hill factors that guide scientists in drawing causal conclusions from epidemiological studies.”). For a detailed analysis of the Milward decision and the weight of the evidence approach to scientific reasoning, see Symposium: Toxic Tort Litigation: After Milward v. Acuity Products, 3 Wake Forest J.L. & Pol’y 1 (2013).
Reliance on Anecdotal Evidence
Courts have generally found that an expert who bases a scientific opinion solely on one or even a few case studies has an insufficient basis for reliable testimony under Rule 702. There are two problems with relying on such anecdotal data, at least exclusively. First, anecdotal evidence is usually derived from insufficient sampling—a handful of select cases that do not reflect a cross section of the relevant population. Second, there is a strong possibility that anecdotal cases are not comparable to the facts of the case at bar. For example, if a number of people contract lung cancer through an alleged exposure to a toxic substance, a proper statistical study of the causative link between the toxic substance and the cancer will exclude the possibility of other sources for the injury (referred to as confounding factors), such as cigarette smoking. But a single case study, or other anecdotal evidence, is unlikely to be screened for confounding factors.
For example, in Cavallo v. Star Enterprises,97 the plaintiff alleged that she suffered respiratory illness as a result of exposure to aviation jet fuel vapors that were released from the defendant’s storage terminal. The plaintiff’s expert toxicologist was prohibited from testifying after a Daubert hearing, and the court consequently granted summary judgment for the defendant. To support an opinion that the plaintiff’s respiratory illness was caused by the jet fuel vapors, the toxicologist relied solely on case studies in which people who were exposed to the organic compounds in jet fuel suffered respiratory illnesses. The court held that reliance on these studies to form a conclusion was inconsistent with the scientific method. The court reasoned that “case reports are not reliable scientific evidence of causation, because they simply describe reported phenomena without comparison to the rate at which the phenomena occur in the general population or in a defined control group; do not isolate and exclude potentially alternative causes; and do not investigate or explain the mechanism of causation.”98
This is not to say that case studies are completely irrelevant to an analysis consistent with the scientific method. The problem in Cavallo was that the case studies were, essentially, the only source upon which the expert based his opinion. In contrast, scientists often use case studies to spur further research or to confirm conclusions reached in more methodical studies.99
97. 892 F. Supp. 756, 767 (E.D. Va. 1995), aff’d in pertinent part, 100 F.3d 1150 (4th Cir. 1996).
98. Id.; see also Norris v. Baxter Healthcare Corp., 397 F.3d 878, 885 (10th Cir. 2005) (“. . . case reports suffer from a similar failing. Case reports that state that some women with breast implants developed disease do not provide an adequate scientific basis from which to conclude that breast implants in fact cause disease. A correlation does not equal causation.”).
99. See, e.g., Wendell v. GlaxoSmithKline LLC, 858 F.3d 1227, 1236–37 (9th Cir. 2017) (“Although case studies alone generally do not prove causation, they ‘may support other proof of causation.’ Here, the experts relied not just on these studies—which not only examined reported cases but also used statistical analysis to come up with risk rates—but also on their own wealth of experience and additional literature.”) (citation omitted); Cantrell v. GAF Corp., 999 F.2d 1007,
Reliance on Temporal Proximity
There are a number of cases in which a healthy plaintiff has become ill shortly after his exposure to a product. For example, in Porter v. Whitehall Lab.,100 Porter came to the hospital to be treated for a fractured toe. He had no other documented health problems. He was given ibuprofen. Over the following month, the plaintiff took about thirty tablets. At the end of that month, the plaintiff was diagnosed with rapidly progressive glomerulonephritis (RPGN), a form of end-stage renal failure from which he would not recover. At the time, no studies demonstrated a link between ibuprofen use and RPGN.101 Porter’s experts, however, testified that ibuprofen caused his RPGN. The experts essentially based their opinions on Porter’s prior good health and the temporal proximity between the ingestion of ibuprofen and his renal failure. The court concluded, however, that forming a conclusion on the basis of temporal proximity alone, in the absence of some established scientific connection between substance and illness, is inconsistent with the scientific method. By relying solely on temporal proximity, the expert fails to consider other possible explanations that a scientist must consider before drawing a conclusion. An expert who determines causation by relying only on temporal proximity commits the classic error of confusing correlation and causation.
But this is not to say that the temporal proximity between exposure and injury can never be relied upon in reaching an opinion on causation. The result in Porter might well have been different if published controlled studies and/or epidemiological evidence established some connection between his ibuprofen intake and RPGN. Then the temporal proximity between exposure and injury could be used as confirmatory data. This was the case in Kennedy v. Collagen Corp., above, where the expert’s opinion on causation was based, in part, on the plaintiff’s having contracted lupus shortly after receiving collagen injections.102 The expert could properly rely on this factor in light of the studies indicating a substantial connection between collagen and the autoimmune antibodies present
1014 (6th Cir. 1993) (expert testified that asbestos created a risk of laryngeal cancer, basing his conclusion on epidemiological evidence reported in the medical literature, and on the inordinately high incidence of persons at the plaintiffs’ workplace whom the expert had personally diagnosed as having laryngeal cancer; the court held that the expert testimony was properly admitted, stating that “[n]othing in Rules 702 and 703 or in Daubert prohibits an expert from testifying to confirmatory data, gained through his own clinical experience, on the origin of a disease or the consequences of exposure to certain conditions”).
100. 9 F.3d 607 (7th Cir. 1993).
101. There were studies suggesting a connection between ibuprofen and interstitial nephritis—a secondary kidney condition from which Mr. Porter also suffered. But there was no demonstrated connection between nephritis and RPGN. Nor did the studies showing a connection between nephritis and ibuprofen intake support causation as a result of the very modest dosage to which Porter was subjected.
102. 161 F.3d 1226, 1230 (9th Cir. 1998).
in lupus patients. As one court put it, “[a] temporal connection is entitled to greater weight when there is an established scientific connection between exposure and illness or other circumstantial evidence supporting the causal link.”103
Insufficient Connection Between the Expert’s Opinion and the Facts in the Case: The Requirement of “Fit”
Some experts may rely on a proper methodology, but then fail to connect the methodology with the facts of the case. This is the problem of “fit.” An example of a failure to connect reliable expert testimony with the facts arose in Harris v. Remington Arms Company, LLC.104 In Harris, the plaintiff brought a product liability action alleging that her Remington rifle had fired and injured her when the safety was turned off—but the trigger was not pulled. The plaintiff offered an expert witness to explain how the rifle could have fired without anyone pulling the trigger. The expert testified that the safety and the trigger mechanism bonded together after the plaintiff had engaged the safety and then stored the rifle in a cold room, causing a liquid bonding agent to solidify. According to the expert, therefore, disengaging the safety caused the trigger to be pulled simultaneously. There was no challenge to the methodology relied upon by the expert to conclude that the liquid bonding agent could solidify in cold temperatures. But this testimony did not fit the facts of the case because the plaintiff had undisputedly disengaged the safety at other times after taking the gun from the cold room, and it had not gone off. The court found that the trial court did not err in excluding the expert’s testimony, because of its “misfit with the undisputed evidence.”105
103. Curtis v. M&S Petroleum, Inc., 174 F.3d 661, 670 (5th Cir. 1999) (abuse of discretion to exclude the testimony of a doctor who concluded that the plaintiffs’ health problems were caused by exposure to benzene; the expert relied not only on the “strong temporal connection between the refinery workers’ exposure to benzene and the onset of their symptoms” but also on extensive scientific literature establishing a connection between benzene and the symptoms suffered by the plaintiffs). See also Claussen v. M/V New Carissa, 339 F.3d 1049 (9th Cir. 2003) (in concluding that damage to oyster beds was caused by an oil spill, the expert could rely on the temporal proximity between the spill and the damage, when combined with field studies, government reports, and exclusion of other obvious causes, to reach a reliable opinion under Daubert).
104. 997 F.3d 1107, 1114 (10th Cir. 2021).
105. Id. (“. . . the district court didn’t reject Mr. Powell’s methodology. The court instead concluded that Mr. Powell’s opinion testimony didn’t fit the undisputed evidence.”); see also Bogosian v. Mercedes-Benz of N. Am., Inc., 104 F.3d 472, 479 (1st Cir. 1997). In that case, the plaintiff was run over by her car after putting it in park and exiting the vehicle. She sought to have an engineer testify about the phenomenon of “false park detent”—where the driver feels as if he or she has put the car in park, without looking at the gear shift, but the car is not actually in park. The court held that this testimony was properly excluded under Daubert because the testimony rested on a factual premise—that
Lack of Testing
One of the factors identified by Daubert for assessing the reliability of an expert’s methodology pursuant to Rule 702 was whether the expert’s technique or theory can be or has been tested. Courts sometimes exclude experts when they have not tested theories or techniques that are capable of being tested. For example, in Truck Insurance Exchange v. MagneTek Inc.,106 the Tenth Circuit held that the trial court properly excluded plaintiff’s expert testimony concerning the novel theory of “pyrolysis.” This theory hypothesized that wood could ignite at temperatures much lower than normal under particular circumstances. The expert testimony was excluded because neither the experts nor others in the scientific community had tested the novel theory sufficiently to demonstrate its scientific reliability. The Tenth Circuit affirmed, explaining that the importance of testing as a factor in determining reliability is at its highest when an expert proposes a theory that modifies otherwise well-established knowledge about regularly occurring phenomenon—such as the temperature at which wood will ignite.107
The problem of lack of testing also appears to arise frequently in product liability cases, where the plaintiff calls an expert to testify that the defendant should have designed a product differently. If the alternative design proposed by the expert has never been made and tested, either by the expert or anyone else, courts are likely to prohibit the expert from testifying that such a design was a viable and safer alternative. As the Seventh Circuit noted in Cummins v. Lyle Indus.,108 a number of factors must go into a reliable conclusion that an alternative design should have been employed, such as the compatibility of the design with existing systems, relative efficiency, maintenance costs, ease of servicing, and the effects of price. As the Cummins court put it: “Many of these considerations are product-and-manufacturer-specific, and most cannot be determined reliably without testing.”109
the plaintiff did not look at the console shift before turning off the car—that was at odds with the plaintiff’s own testimony: “The district court appropriately found it very odd that Bogosian would present an expert witness who would testify that her own unwavering testimony was incorrect.”
106. 360 F.3d 1206, 1211–12 (10th Cir. 2004).
107. See also United States v. Gabaldon, 389 F.3d 1090, 1099 (10th Cir. 2004) (testimony of an accident reconstructionist in a murder case that the internal geometry of a car would have made it difficult for the defendant to reach the victim with enough power to inflict blows was properly excluded where he conceded that his theory could have been tested by placing defendant in the car and taking measurements, but that no such testing was done). But see Bitler v. A.O. Smith Corp., 400 F.3d 1227, 1235 (10th Cir. 2004) (expert opinion as to cause of explosion was properly permitted; the fact that the expert did not test his theory about the cause of the gas leak at issue was not dispositive where his opinion was based upon “known science of copper sulfide particulate contamination as a cause of propane gas leaks”).
108. 93 F.3d 362, 369 (7th Cir. 1996).
109. Id. at 370–71 (affirming exclusion of alternative design testimony because the alternative design had not been tested, and the expert’s opinions lent themselves to testing and substantiation by the scientific method).
But the Daubert factors are flexible, and Rule 702 is context-specific. There is no absolute requirement that a conclusion or theory be tested to be admissible. In the specific area of design safety, the feasibility of testing and the expense of litigation must be taken into account. For example, a design engineer may seek to testify that a car could have been designed more safely. A court should not exclude the testimony solely because the expert has not built a car employing the alternative design. A good example of the proper approach to testing after Daubert is found in Tassin v. Sears, Roebuck & Co.110 Tassin was injured while operating a power saw and proffered an engineer who concluded that alternative designs were safer and that the defendant failed to provide adequate warnings. The district court analyzed the admissibility of this testimony as follows:
It may well be that an engineer is able to demonstrate the reliability of an alternative design without conducting scientific tests, for example, if he can point to another type of investigation or analysis that substantiates his conclusions. For example, an expert might rely upon a review of experimental, statistical, or other technical industry data, or on relevant safety studies, products, surveys, or applicable industry standards. He could also combine any one or more of these methods with his own evaluation and inspection of the product based on experience and training in working with the type of product at issue. The expert’s opinion must, however, rest on more than speculation, he must use the types of information, analyses and methods relied on by experts in his field, and the information that he gathers and the methodology that he uses must reasonably support his conclusions. If the expert’s opinions are based on facts, a reasonable investigation, and traditional technical/mechanical expertise, and he provides a reasonable link between the information and procedures he uses and the conclusions he reaches, then [the opinion may be admitted despite a lack of testing].111
Applying these standards to the facts, the Tassin court excluded the expert’s testimony on certain alternative designs based on parts that he had never tested or even seen, and the safety of which was not supported by the tests of others or by any relevant literature. However, the court held testimony as to another alternative design admissible, where the expert had actually conducted some testing and where the safety of the product received support from the relevant literature. After finding sufficient reliable support to admit this opinion under Rule 702, the court explained that the expert’s failure to test more systematically and extensively presented a question of the weight to be afforded to the opinion. Finally, the court held the expert’s conclusion as to inadequate warnings to be admissible. The alternative warnings suggested by the expert had not been scientifically tested. But the court found that testing as to warnings (as opposed to testing alternative designs) was not critical where the expert had
110. 946 F. Supp. 1241, 1247–48 (M.D. La. 1996).
111. Id.
substantial experience in both product design and in preparing product manuals and warnings.
The Problem of Overstatement
Some expert witnesses may utilize a methodology that is sufficiently reliable and that may be applied helpfully to the facts of a particular case but may err in their application by overstating the findings that the methodology is capable of producing (particularly during trial testimony). Overstatement is especially endemic in the testimony of forensic experts. Over the past two decades, various scientific bodies have raised serious questions about the reliability of forensic techniques that had theretofore been admitted without much controversy. The two most important reports are:
- National Research Council of the National Academy of Sciences (NAS), Strengthening Forensic Science in the United States: A Path Forward (2009). This detailed report essentially concludes that with one exception, none of the forensic techniques currently employed—such as ballistics, handwriting, and hair identification—meet the standards of science. The exception is identification of DNA from a single source.
- The President’s Council of Advisors on Science and Technology (PCAST), Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods (2016). This report provides an exhaustive analysis of why forensic feature comparison methods (specifically ballistics, handwriting, fingerprinting, footwear, hair, and DNA analysis from complex mixtures) have not been validated, and how at least some of them can be strengthened so that they have validity. Particular attention is given to the problem of experts overstating their results—e.g., making statements that fingerprint identification has a “zero rate of error” when in fact the process is based on subjective judgment that by definition carries a rate of error.
Despite these important reports, most courts have admitted forensic expert opinions as reliable technical testimony under Kumho even though they are not based on the scientific method.112 Some courts, though, have rightly focused on
112. See, e.g., United States v. Baines, 573 F.3d 979, 989 (10th Cir. 2009) (conceding that there are “multiple questions regarding whether fingerprint analysis can be considered truly scientific in an intellectual, abstract sense” but concluding that “nothing in the controlling legal authority we are bound to apply demands such an extremely high degree of intellectual purity” and that “fingerprint analysis is best described as an area of technical rather than scientific knowledge”); United States v. Johnson, 2019 U.S. Dist. LEXIS 39590 (S.D.N.Y.) (finding ballistics identification to be admissible, and claiming that the NAS and PCAST reports impose “demanding scientific standards” that “require a level of certainty and infallibility not properly applied in a courtroom”).
preventing forensic experts from overstating their results—for example, prohibiting such experts from testifying that they are “certain” that there is a “match,” or that the rate of error is “zero” or “infinitesimal.” The leading case on preventing overstatement is United States v. Glynn,113 in which the district court found that because ballistics identification is based on subjective judgment, the expert could not testify that he was a “scientist” or that the rate of error for his identification was zero. The court allowed the expert to testify but held that he could state only that it was “more likely than not” that the bullet fragment came from the defendant’s gun—as that was all the certainty that the subjective methodology could support. The trial judge explained that it is particularly important for judges to regulate expert overstatement as part of the gatekeeping role because:
[O]nce expert testimony is admitted into evidence, juries are required to evaluate the expert’s testimony and decide what weight to accord it, but are necessarily handicapped in doing so by their own lack of expertise. There is therefore a special need in such circumstances for the Court, if it admits such testimony at all, to limit the degree of confidence which the expert is reasonably permitted to espouse.114
While some other courts have followed suit and have sought to control overstatement in expert testimony,115 many others have not.116 Because the problem of overstatement has not been well regulated by the courts, the Advisory Committee on Evidence Rules, starting in 2017, began considering whether Rule 702
113. 578 F. Supp. 2d 567, 575 (S.D.N.Y. 2008).
114. Id.
115. See, e.g., United States v. Machado-Erazo, 901 F.3d 326 (D.C. Cir. 2018) (error for a cell phone location expert to testify to the precise location of the defendant’s phone, where the current technology can locate a phone only within a general area); United States v. Parker, 871 F.3d 590 (8th Cir. 2017) (trial court prohibited the expert from testifying that she was “100% sure” or “certain” that the relevant guns matched the relevant shell casings); United States v. Hill, 818 F.3d 289, 295 (7th Cir. 2016) (declaring that cell site analysis expert testimony should include a “disclaimer” regarding accuracy; the expert should not “overpromise on the technique’s precision or fail to account for its flaws”; cell site testimony was admissible where the agent testified that the defendant’s phone records were “consistent” with him being at or near relevant locations at relevant times, but clarified that he could not state whether a phone was “absolutely at a specific address”); United States v. White, 2018 U.S. Dist. LEXIS 163258 (S.D.N.Y.) (finding proposed ballistics testimony was admissible, subject to the limitation that the expert could not testify to any specific degree of certainty that there was a ballistics match between the firearms seized from the defendant and those used in the various shooting incidents).
116. See, e.g., United States v. Straker, 800 F.3d 570 (D.C. Cir. 2015) (fingerprint expert allowed to testify that there was a “zero rate of error in the methodology”); United States v. Hunt, 2020 WL 2842844 (W.D. Okla. 2020) (the defendant asked the court “to place limitations on the Government’s firearm toolmark experts because the jury will be unduly swayed by the experts if not made aware of the limitations on their methodology”; the court allowed the expert to testify to a “reasonable degree of certainty” even though the Department of Justice standards do not permit its experts to testify in those terms).
should be amended to prohibit overstatement by experts. After careful consideration, the committee concluded that overstatement was already within the court’s gatekeeping responsibility under Rule 702, as amended in 2000, because an expert who overstates a conclusion is not reliably applying her basis and methodology. But the committee determined that the obligation to regulate overstatement should be highlighted for the courts with a modest amendment to Rule 702(d). The 2023 amendment to Rule 702(d) discussed above requires the court to find that “the expert’s opinion reflects a reliable application of the principles and methods to the facts of the case.”117 The amendment instructs trial courts to carefully evaluate the actual opinion rendered—the opinion itself and not just the process of applying a methodology—to make sure that it does not go beyond what the expert can reliably conclude. The Committee Note to the 2023 amendment emphasizes that the focus of the amendment is on preventing experts from overstating their opinions:
Rule 702(d) has . . . been amended to emphasize that each expert opinion must stay within the bounds of what can be concluded from a reliable application of the expert’s basis and methodology. Judicial gatekeeping is essential because just as jurors may be unable, due to lack of specialized knowledge, to evaluate meaningfully the reliability of scientific and other methods underlying expert opinion, jurors may also lack the specialized knowledge to determine whether the conclusions of an expert go beyond what the expert’s basis and methodology may reliably support.
While the Committee Note voices objection to testimony about a “zero rate of error,” the intent of the amendment is to require court scrutiny of any attempt by a forensic expert to testify to greater certainty than is supported by the methodology. A common and problematic form of expert overstatement occurs when an expert witness purports to testify “to a reasonable degree of scientific certainty.” Expert overstatement was a significant focus of the PCAST report on forensic feature-comparison methods.118 In particular, the PCAST report recommended that courts “should never permit scientifically indefensible claims, such as . . . proof to a reasonable degree of scientific certainty.”119 A report from the National Commission on Forensic Sciences similarly decried this type of expert overstatement and proposed that courts should forbid experts from stating their conclusions to a “reasonable degree of [field of expertise] certainty,” because that term has no scientific foundation. The Department of Justice (DOJ) has adopted testimonial protocols that prohibit the use of the “reasonable degree of certainty” language for certain feature-comparison experts, and that impose important limitations on testimony regarding rates of error. For example, a DOJ
117. The amendment took effect on December 1, 2023.
118. President’s Council of Advisors on Science and Technology, Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods. (Sept. 20, 2016).
119. Id. at 145.
directive regarding toolmark testimony provides, in pertinent part: “An examiner shall not use the expressions ‘reasonable degree of scientific certainty,’ ‘reasonable scientific certainty,’ or similar assertions of reasonable certainty in either reports or testimony, unless required to do so by a judge or applicable law.”120 It is for this reason that the Advisory Committee Note to the 2023 amendment to Rule 702 specifically explains: “Forensic experts should avoid assertions of absolute or one hundred percent certainty—or to a reasonable degree of scientific certainty—if the methodology is subjective and thus potentially subject to error.” Courts should thus prohibit, where possible, the use of problematic testimony to a “reasonable degree of scientific certainty” notwithstanding significant past federal precedent in which such testimony was permitted or even required.121
Experts Relying on Technical or Other Specialized Knowledge
As discussed above, the Supreme Court’s Kumho decision clarified that Rule 702 and the Daubert reliability standard apply to expert opinion testimony based upon technical or other specialized knowledge, as well as to scientific testimony. Even though the gatekeeper function applies to nonscientific expert testimony, it stands to reason (as the Court in Kumho recognized) that the specific Daubert factors will often have to be rejected or modified when reviewing most nonscientific expert testimony. A court should not exclude an expert on automobile repair on the ground that he has not published his research in a peer-reviewed journal. A court should not reject the testimony of a sociologist simply because she cannot be definitive about a possible rate of error in her findings.
120. United States Department of Justice Uniform Language for Testimony and Reports for the Forensic Firearms/Toolmarks Discipline Fracture Examination 3–4, https://perma.cc/P6D4-BCC9.
121. There can be a complication in rejecting the reasonable degree of certainty standard in civil cases, however. Some states appear to require a reasonable certainty standard as a matter of state substantive law—which is controlling in diversity cases, assuming that in fact it is substantive. See, e.g., Miranda v. County of Lake, 900 F.3d 335 (7th Cir. 2018) (“In Illinois, proximate cause must be established by expert testimony to a reasonable degree of medical certainty.”); Day v. United States, 865 F.3d 1082 (8th Cir. 2017) (under Arkansas law, a medical expert must testify that “the damages would not have occurred” without the defendant’s negligence; expert’s opinion “must be stated within a reasonable degree of medical certainty or probability”). For this reason, the advisory committee’s note to the 2023 amendment to Rule 702 explains that the “amendment does not, however, bar testimony that comports with substantive law requiring opinions to a particular degree of certainty.”
An example of this necessary flexibility is found in Tyus v. Urban Search Management.122 The plaintiffs in Tyus alleged that advertising for a rental building violated the Fair Housing Act because it targeted only white lessees. The plaintiffs proffered a social science expert who would have testified to how an all-white advertising campaign affects African Americans. The court declared that the central teaching of Daubert—that expert testimony “must be tested to be sure that the person possesses genuine expertise in a field and that her court testimony adheres to the same standards of intellectual rigor that are demanded in her professional work”—was fully applicable to the testimony of experts in the social sciences. However, recognizing the need for a flexible application of Daubert, the court noted the following caveat:
It is true, of course, that the measure of intellectual rigor will vary by the field of expertise and the way of demonstrating expertise will also vary. Furthermore, we agree . . . that genuine expertise may be based on experience or training. In all cases, however, the district court must ensure that it is dealing with an expert, not just a hired gun.
The Tyus court found that the trial court erred in excluding the expert, because his research was based on peer-reviewed articles, and his “focus group” method was a well-accepted methodology in the field of social science. In other words, the expert brought the same intellectual rigor to his courtroom testimony as he employed in his life as a social scientist.
Where expert testimony is not adaptable to being put through the wringer of falsifiability, publication, and peer review, the trial judge still has the obligation to determine whether the testimony is properly grounded, well reasoned, and not speculative. If there is a well-accepted body of learning, experience, and basic assumptions in the field, then the expert’s testimony should be grounded in that learning and experience to be reliable. The more subjective and controversial the expert’s inquiry, the more likely the testimony is to be excluded as unreliable. The opinion in Frymire-Brinati v. KPMG Peat Marwick123 is instructive. In this action for securities fraud, the plaintiff’s expert, an accountant, was allowed to testify that a Peat Marwick audit had improperly certified that property interests had a certain value when in fact they were worth much less. To reach this conclusion, the accountant used a discounted cash flow analysis, by which he assessed value solely on the basis of net, rather than potential, cash flow. He did not explain why he failed to consider potential cash flow, even though standard principles of valuation find it to be an important factor. Reversing a judgment for the plaintiff, the court held that the trial court abused its discretion in admitting the expert’s valuation, because the expert’s
122. 102 F.3d 256, 263 (7th Cir. 1996).
123. 2 F.3d 183, 188 (7th Cir. 1993).
methodology was faulty: by failing to consider potential cash flow, the expert’s methodology would lead to the conclusion that “raw land is worthless and that a large office building in the final stages of construction also has no value even though it is fully leased out and could be sold for a hundred million dollars.” The Seventh Circuit held that the expert’s testimony lacked validity under Daubert, where his litigation methodology would not have been acceptable for clients outside the courtroom.
A particular problem after Kumho involves the nonscientific expert who testifies solely on the basis of experience. The example from Kumho is the perfume tester who sniffs a substance and identifies its brand and provenance. When asked how he can make such a specific identification, he responds, “I know it when I smell it and I smell it a lot.” Should he be permitted to testify solely on this conclusory experience-based analysis? Put another way, must the trial court take the expert’s word for it? The Court in Kumho implies that it is inconsistent with the gatekeeper function to allow an experience-based expert to testify without any explication of how he or she comes to a conclusion and why he or she believes it is correct. This implication—that experience-based experts must justify their conclusions as reliable—was codified in the 2000 amendment to Rule 702.
For example, in United States v. Rodriguez,124 law enforcement officers testified as experts interpreting intercepted phone calls. But the officers testified about uncommon terms or phrases that they encountered for the first time in the investigation. They relied on experience, but the court found error in admitting their testimony, because “the officers generally did not offer an explanation for how they arrived at their interpretations, nor did the court require them to provide one.” Any explanations the officers did offer for arriving at their interpretations “failed to evince indicia of reliability or methodological rigor.” The court noted that “an officer’s qualifications, including his experience with investigations and intercepted communications, are relevant but not alone sufficient to satisfy Federal Rule of Evidence 702” because “Rule 702 requires district courts to assure that an expert’s methods for interpreting the new terminology are both reliable and adequately explained.”125 To be sure, law enforcement officers are frequently permitted to give expert testimony regarding the meaning of code words, particularly in gang- and drug-related cases, so long as they offer the requisite explanation of their methodology.126
124. 971 F.3d 1005, 1018 (9th Cir. 2020).
125. Id.; see also United States v. Hermanek, 289 F.3d 1076 (9th Cir. 2007) (expert testimony translating purported coded conversation should have been excluded; although the government made an adequate showing of the witness’s experience, the witness did not explain in detail the methods he used to arrive at his interpretation of words that he was not familiar with before the investigation).
126. See United States v. Wilson, 484 F.3d 267, 275 (4th Cir. 2007) (explaining that “courts of appeals have routinely held that law enforcement officers with extensive drug experience are
Conclusion
Exercising the gatekeeping function under Daubert and Rule 702 is sometimes daunting and often requires a substantial expenditure of judicial resources. But the alternative—leaving questions of expert reliability to the jury—is inconsistent with Rule 702 and established Supreme Court precedent. Thus, the trend over time has been for judges to exercise closer scrutiny of expert testimony, and this trend may be expected to continue as evidence in litigation becomes more complex.
qualified to give expert testimony on the meaning of drug-related code words” and affirming admission of such testimony where expert carefully “summarized his methodology”).
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