When tasked with assessing the admissibility of expert testimony under Federal Rule of Evidence 702, courts often cite the so-called Daubert factors as criteria that guide the inquiry. Among those factors is “whether the [expert’s] theory or technique has been subjected to peer review and publication.” Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579 (1993). The Daubert Court observed that, although publication “is not a sine qua non of admissibility,” peer review “increases the likelihood that substantive flaws in methodology will be detected.” But peer review is not coterminous with the Rule 702 inquiry that federal courts are called upon to make, especially with the rise of so-called predatory publishing and journals with relaxed (or absent) peer review processes. As one court recently observed, “a court can’t wave junk science through the Daubert gate simply because it survived some prepublication peer-review process.” In re: Roundup Products Liability Litigation, 2024 WL 3074376 (N.D. Cal. June 20, 2024).
In In re: Roundup, the plaintiff claimed to have developed non-Hodgkin’s lymphoma (NHL) as a result of using the defendant’s herbicide. In support of that claim, he offered a single expert on the issue of whether glyphosate, the active ingredient in the herbicide, can cause NHL in humans. The expert’s opinions were all contained in two peer-reviewed and published articles that the expert had co-authored. But only one of the two—a 2019 meta-analysis of six epidemiological studies addressing the link between glyphosate and NHL, which had been published before the expert became involved in the litigation—grappled with the available epidemiological evidence. The defendant attacked that paper on multiple grounds, and the court agreed that it constituted “junk science” with several flaws each independently justifying its exclusion.
The court first determined that the meta-analysis was “not reliably performed” and had “several methodological problems that, when taken together, make its analysis of the epidemiological literature indefensible.” After meticulously reviewing the details of both the meta-analysis and the underlying studies it analyzed, the court concluded that the meta-analysis restricted itself to an “arbitrary subset of the epidemiological data points” by failing to isolate relevant high-exposure data, mixing whole data sets with partial ones, blending adjusted data with unadjusted data, and using only a small portion of the data from the highest-quality available study.
The plaintiff predictably protested that the meta-analysis had the standard indicia of reliability under Daubert because it was published in a peer-reviewed journal, was produced independently of litigation, had been cited many times, and resulted in the expert being invited to present at various scientific societies. But, as the court noted, “while those facts are entitled to consideration, they don’t mean that blind deference to [the expert] is appropriate.” “Pre-publication editorial peer review, just by itself, is far from a guarantee of scientific reliability.”
The court also observed that even if the meta-analysis had been reliably performed, “a reliable meta-analysis is not the same thing as a reliable general causation opinion.” Although the meta-analysis claimed “a compelling link” between exposure to glyphosate-based herbicides and an increased risk for NHL, it did not go on to suggest that exposure to glyphosate can cause NHL. That is, “saying there is a ‘compelling link’ or an ‘association’ isn’t the same as saying that [the herbicide at issue] is capable of causing NHL in humans—which is what a plaintiff’s general causation evidence must enable the jury to conclude.” The court also noted that the meta-analysis was dated, admonishing that “an expert testifying in 2024 couldn’t reliably opine that glyphosate is carcinogenic if they only considered epidemiological evidence published before 2019.”
Moreover, the court was perturbed by the expert’s inability at the hearing to “answer basic questions about her meta-analysis or the handful of studies it incorporated.” For example, the expert could not remember the names of all six studies analyzed and could not remember important facts about how those studies were conducted. The expert’s “lack of familiarity with her own opinion is itself a basis to exclude her testimony.”
Peer review remains a helpful indicator of reliability to the extent that it suggests a paper has survived scrutiny by experts in the relevant field. But peer review is merely one factor to consider in the Rule 702 inquiry, and even peer-reviewed publications are subject to judicial scrutiny. As the In re: Roundup court observed, where a court “identif[ies] serious problems in an expert opinion, it doesn’t make sense for the Court to ignore them” even if those opinions have survived peer review.
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