One of the most frequent lines of attack against plaintiffs’ causation experts in a Daubert challenge is the failure to rule out potential alternative causes that could independently explain the plaintiffs’ alleged injuries. Rooted in longstanding Daubert jurisprudence, this argument often succeeds, but has faced challenges in certain courts, including some within the Eighth Circuit, recognizing that such failures go to the weight of the opinions, as opposed to the admissibility. A recent Eighth Circuit opinion provides clarity regarding when plaintiffs’ experts must consider potential alternative causes in forming their opinions, and when their failure to do so could render their opinions inadmissible under Daubert. See Redd v. DePuy Orthopaedics, Inc., No. 16-3428, 2017 U.S. App. LEXIS 11930 (8th Cir. July 5, 2017).
Daubert motion practice in the Eighth Circuit has been in flux since the Eighth Circuit reversed a district court’s Daubert order in Johnson v. Mead Johnson & Co., LLC, 754 F.3d 557 (8th Cir. 2014). There, the plaintiff’s expert opined that a child had developed an extremely rare type of infection as a result of his consumption of a baby formula known to have been contaminated with a microbe associated with the same rare infection. Moving to exclude the opinion, the defendant argued that the same infection-causing microbe could be found in the plaintiff’s tap water and was frequently found in ordinary household kitchens—potential alternative sources of the infection that the plaintiff’s expert did not rule out. The district court granted the defendant’s Daubert motion because the expert failed to rule out these alternative causes. The Eighth Circuit reversed, reasoning that “we have consistently ruled that experts are not required to rule out all possible causes when performing the differential etiology analysis,” and that “[i]nstead, such considerations go to the weight to be given the testimony by the factfinder, not its admissibility.” 754 F.3d at 563-64.
Since the Eighth Circuit’s decision in Johnson, there have been few cases excluding an expert’s opinions for failure to rule out alternative causes. Some plaintiffs have even argued that Johnson disposed of any such requirement. Earlier this month, however, the Eighth Circuit clarified the important role this requirement plays in the Daubert analysis, and confirmed that an expert’s failure to consider alternative causes—at least the “obvious” ones—can be fatal to the admissibility of his or her opinions. Redd, 2017 U.S. App. LEXIS 11930.
In Redd, the plaintiff underwent a total hip replacement and subsequently sustained a fracture of the implanted hip stem. She underwent a revision surgery, and the second hip stem fractured in a similar manner. The plaintiff’s expert opined that manufacturing defects caused each of the implants to fracture. While the expert acknowledged that environmental factors—i.e., the biomechanical forces to which the stems were subject after being implanted—could also have contributed to the failure, he contended, without analysis, that such factors would have been secondary to the alleged manufacturing defects at best. The district court excluded the expert’s opinions as to defect and causation, because even though he acknowledged extraneous biomechanical forces could have contributed to the product failure, he did not measure any such forces or rule them out as a cause. On appeal, the Eighth Circuit affirmed, noting that “[a]lthough Redd is correct that an expert need not rule out all possible causes of an injury, an expert nonetheless should ‘adequately account for obvious alternative explanations.’” 2017 U.S. App. LEXIS 11930, at *7 (citations omitted). Because the plaintiff’s expert did not consider obvious alternative explanations offered by the plaintiff’s treating physicians, let alone the alternatives that he himself acknowledged, the Eighth Circuit affirmed the district court’s Daubert order.
Redd is therefore the latest significant Daubert decision out of the Eighth Circuit, clarifying the role of alternative cause analysis in determining the reliability of an expert’s methodology. Whereas before, plaintiffs could (and did) lean on the 2014 Johnson opinion to argue that their experts need not rule out alternative causes, Redd now makes clear that plaintiffs’ experts must address potential alternative causes head on—at least those that are “obvious.” With any luck, the Redd decision will lay to rest this common misconstruction of an expert’s obligations under Daubert and Rule 702.