Several recent decisions have shed light on the courts' willingness to dismiss a product liability action where the plaintiff lacks sufficiently reliable evidence of general causation'that is, evidence that the product can cause the purported negative outcome.
Two related scientific concepts frequently arise, especially in cases involving pharmaceuticals or chemicals: (i) relative risk rates1 and (ii) the Bradford-Hill criteria for general causation. It is important for defense counsel to understand these concepts so that they can identify the best arguments for dismissal, particularly at the Daubert stage.
RELATIVE RISK RATIOS
Relative risk was succinctly described by the Northern District of Florida in a 2018 order in In re Abilify: "Relative risk is simply a comparison of the incidence of a disease in exposed individuals with its incidence in unexposed individuals."2 In the pharmaceutical context, relative risk can be determined by tracking and comparing the rate of negative outcomes over a given period of time in two groups of patients'those who are taking a drug that allegedly causes the negative outcome and those who are not taking the drug.3 This comparison of risks results in a risk ratio, which is calculated by dividing the rate of negative outcomes in the exposed group by the rate of negative outcomes in the non-exposed group.4
For instance, as the Central District of California explained in its 2004 In re Silicone Gel Breast Implants opinion:
[I]f a study found that 10 out of 1000 women with breast implants were diagnosed with breast cancer and 5 out of 1000 women without implants (the "control" group) were diagnosed with breast cancer, the relative risk of implants is 2.0, or twice as great as the risk of breast cancer without implants. This is so, because the proportion of women in the implant group with breast cancer is 0.1 (10/1000) and the proportion of women in the non-implant group with breast cancer is 0.05 (5/1000). And 0.1 divided by 0.05 i5
Thus, a relative risk of 1.0 indicates that there is no difference in the rate of negative outcomes between the exposed and non-exposed groups, meaning that the exposure and the negative outcome have no association.6 A relative risk of less than 1.0 indicates that fewer people in the exposed group experienced the negative outcome, suggesting a potential protective effect.7 A relative risk above 1.0 indicates that the exposed group experienced negative outcomes at a greater rate than the non-exposed group, suggesting that the exposure may be associated with an increased risk of the outcome in question.8
Importantly though, association is not causation. As the Southern District of New York explained in its September 2021 Daniels-Feasel opinion, a reliable causation opinion must go beyond epidemiological studies showing a statistically significant increased risk to further "assess whether an exposure-disease relationship is merely associative or is in fact causal."9 Thus, to determine the existence of a causal relationship, epidemiologists commonly analyze the relevant body of scientific evidence and data "using the so-called 'Bradford Hill' criteria."10 Thus, the relative risk reported by scientific studies is relevant to a Bradford-Hill analysis but is only part of the overall assessment.
THE BRADFORD-HILL CRITERIA
The overall general causation inquiry is typically guided by an analysis of nine-factors known as the Bradford-Hill criteria.11 As summarized in the recent Daniels-Feasel opinion, the Bradford-Hill criteria are:
- Strength of...
