When does an expert witness have an obligation to weigh competing studies and explain why she chose to rely on one study rather than another? On the one hand, this decision-making process goes to the core of whether an expert has employed a sound methodology in reaching her conclusions—a requirement that district courts must police under Daubert. But on the other hand, the selection of studies could be viewed as going to weight, not admissibility, and thus the jury, not the district court or the expert, should decide which study to credit.

This question was at the center of a recent First Circuit decision. InMilward v. Rust-Oleum Corp., ___ F.3d ___, 2016 WL 1622620 (Apr. 25, 2016), a divided panel held that the district court did not abuse its discretion in excluding expert testimony on the ground that the expert failed to analyze conflicting epidemiological studies.

The plaintiff worked as a pipefitter and refrigerator technician for over 30 years. During that period, he was exposed to benzene—estimated at 25.6 ppm-years by the plaintiff’s expert—from paints manufactured by the defendant. In 2004, the plaintiff was diagnosed with Acute Promyelocytic Leukemia (APL) and subsequently sued the defendant on a negligence theory of liability.

To prove specific causation—i.e., that exposure to benzene was a substantial factor in the development of his APL—the plaintiff sought to introduce expert testimony from Dr. Sheila Butler, a Veterans Administration employee who specializes in clinical assessments of environmental and occupational exposure in veterans. Dr. Butler presented three theories of liability.

First and most prominently, Dr. Butler opined that there is no safe level of benzene exposure and, given this “no-safe level” theory, argued that benzene was the likely cause of the plaintiff’s APL. The district court rejected this theory on the ground that it could not be properly verified, and the plaintiff did not contest this ruling on appeal.

Second, Dr. Butler relied on a study showing that workers exposed to benzene levels of 8 ppm-years or more were seven times more likely than the control group to develop APL. Relying on this relative-risk theory and the fact that the plaintiff’s exposure was 25.6 ppm-years, Dr. Butler reasoned that benzene caused the plaintiff’s APL.

But at her deposition, Dr. Butler was confronted with a different study finding no increased risk of APL among workers exposed to less than 40 ppm-years of benzene. When asked whether she intended to weigh the conflicting studies, Dr. Butler responded “no” and stated that she was relying on what she knew about biology and pathophysiology. The district court rejected Dr. Butler’s relative-risk theory of specific causation because she had “expressly disavowed her intent, and minimized her ability, to analyze conflicting epidemiological studies.”

Finally, Dr. Butler conducted a differential diagnosis by ruling out common causes of APL, such as obesity and smoking. In other words, Dr. Butler “ruled in” benzene as the cause of the plaintiff’s APL because she had ruled out other known causes. The district court declined to credit Dr. Butler’s differential diagnosis because it was “circular” and because approximately 70-80 percent of APL diagnoses are idiopathic—that is, without a known cause.

The district court accordingly excluded Dr. Butler as an expert witness and granted summary judgment to the defendant.

A divided panel of the First Circuit affirmed. Chief Judge Howard, joined by District Judge Laplante, rejected the plaintiff’s challenge to the district judge’s relative-risk ruling. The plaintiff argued that the studies were not, in fact, contradictory because the 40 ppm-years study did notaffirmatively find the absence of a relationship. The First Circuit disagreed with this argument, explaining that Dr. Butler’s choice between the two studies “yield[ed] a vastly different comparison,” and the district court did not clearly err in finding that this choice necessarily led to a different conclusion. The First Circuit also rejected the plaintiff’s fact-bound contention that the district court misconstrued Dr. Butler’s answer by exaggerating the extent to which she disavowed her ability to weigh competing studies. And most significantly for its relative-risk holding, the First Circuit concluded that “where an expert’s medical opinion is grounded exclusively on scientific literature, a district court acts within its discretion to require the expert to explain why she relied on the studies that she did and, similarly, why she disregarded other, incompatible research.”

Finally, the First Circuit affirmed the district court’s ruling as to Dr. Butler’s differential diagnosis on the ground that Dr. Butler only “ruled in” benzene by relying on her two other, discredited theories.

Judge Thompson dissented on the relative-risk holding. After providing a useful primer on expert opinions, Judge Thompson delved into the district court’s ruling. She faulted the district judge for overemphasizing the fact that Dr. Butler was “not an epidemiologist” because, under First Circuit precedent, an expert need not be a specialist in a specific medical discipline to provide expert testimony on that subject. Judge Thompson then turned to the relative-risk studies, agreeing with the plaintiff that the two studies were not in conflict because the 40 ppm-years study did not find that “benzene-exposure levels of 25.6 ppm-years or lower cannot cause leukemia.” But even assuming a conflict, Judge Thompson maintained that the district judge “misread[] [Dr. Butler’s] deposition” and erred by finding that Dr. Butler had refused to explain her reasoning. According to Judge Thompson, Dr. Butler’s reliance on her personal experience in biology and pathophysiology—as opposed to a reading of the scientific literature—“square[s] with [the] caselaw.” And, in the most important clash with the majority, Judge Thompson argued that experts “are not reflexively obliged to ‘discredit’ a study pushed by their opponents.” Rather, she argued, the expert needs only to show that she reached her conclusion “via a sound methodology,” and the jury should get to decide which study, if any, it credits.

But Judge Thompson lost the argument. As a result, expert witnesses in the First Circuit will have to be much more punctilious about considering the full range of studies when rendering their opinions. Those who cherry-pick their studies in an effort to support an opinion favorable to the party who retained them risk having their testimony excluded. And plaintiffs who rely on such experts risk having their claims thrown out at the summary-judgment stage.