On April 25, 2016, in Milward v. Rust-oleum Corp., 820 F.3d 469 (1st Cir. 2016), the U.S. Court of Appeals for the First Circuit affirmed the district court’s ruling to reject the relative risk testimony of the plaintiff’s specific causation expert under Rule 702 of the Federal Rules of Evidence.
In this benzene-exposure matter, the plaintiff’s expert concluded that even beyond the no-safe-level hypothesis, certain epidemiological studies have established that an individual's "relative risk" of developing acute promyelocytic leukemia (APL) increases when exposed to specified amounts of benzene. She then compared the plaintiff’s exposure levels to those that had been found to be dangerous in that research. Since the plaintiff’s exposure was higher than the amounts found to be hazardous, the expert reasoned that benzene exposure was likely the cause of the plaintiff’s APL. Notably, she did not explain why she chose the studies on which she relied, nor did she address any study with contrary findings. In fact, during the expert’s deposition, defendant's counsel asked the expert number of questions about her ability and willingness to engage with the relevant epidemiological research. For instance, counsel asked, "Are you aware of any studies which find that there is no relationship between benzene exposure and APL," to which she answered "Yes . . . the literature has support for both." Counsel then asked, "Do you intend in this case to weigh the different epidemiological studies and offer an opinion as to which ones we should rely on and which ones we should discount," to which she replied, "No."
The district court rejected the expert’s relative-risk testimony because the expert had expressly disavowed her intent, and minimized her ability, to analyze conflicting epidemiological studies, and reasoned that, without such analysis, it was impossible to ensure that the studies she cited were actually based on a reliable methodology.
The plaintiff challenged this decision by asserting that the district court relied on an incorrect premise: that conflicting epidemiological studies existed. The plaintiff noted that studies existed establishing an increased risk of APL after a certain level of exposure, such as 8 ppm-years, and also acknowledged that other studies found no increased risk of leukemia with exposure at any level less than 40 ppm-years. They argue, however, that because this first study did not affirmatively find the absence of a relationship, the studies were not actually in conflict. The plaintiff also argued that because the expert did not actually disavow her willingness to consider the divergent studies, the expert’s testimony was still based on reliable evidence and was therefore admissible.
The First Circuit ultimately affirmed the lower court’s decision to reject the expert’s testimony and explained, generally, 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 the expert relied on the studies that the expert did and, similarly, why the expert disregarded other, incompatible research. See, e.g., Kuhn v. Wyeth, Inc., 686 F.3d 618, 623–24 & 633 (8th Cir. 2012)(permitting testimony in which the expert witness relied on methodologically reliable studies and provided an explanation for why those studies were chosen). The court further noted that, when an expert engages in a relative risk analysis in the manner done in this case, the district court is on firm ground in requiring such an explanation, because the validity of the approach depends on the reliability of the studies chosen. That is, if the expert is comparing the plaintiff's condition to a study, and the study is based on an unreliable methodology, then the comparison itself is futile.
The court also concluded that the relevant studies at issue in this case were not only in tension with one another, but expressly cast each other into doubt. See, e.g., EPA Office of Research and Development, Carcinogenic Effects of Benzene: An Update, at 14 (Apr. 1998). Given that, the district court reasonably ruled that there needed to be some indication of why the expert used the studies that she did. Indeed, her complete unwillingness to engage with the conflicting studies (irrespective of whether she was able to or not) made it impossible for the district court to ensure that her opinion was actually based on scientifically reliable evidence and, correspondingly, that it comported with Rule 702.