Proving Causation: Gold Standard Not Required

In Miller v Merck Frosst Canada Ltd. the British Columbia Court of Appeal addressed what it described as its “two most important, recent, and somewhat contrasting decisions”: Stanway, where the class was certified; and Charlton, where it was not. At issue in both decisions was the threshold at the certification stage for evidence of a methodology to prove causation. The Court in Miller concluded that Stanway and Charlton are not in fact at odds with each other. Rather, the type of evidence (whether expert or not) required to overcome “the common issue methodology hurdle” must be considered on a case-by-case basis.


The plaintiffs commenced a class action against Merck relating to the drugs Propecia and Proscar. Propecia is used to treat male pattern baldness and Proscar is used to treat prostate problems; both contain finasteride. The plaintiffs alleged that Merck was negligent in failing to warn consumers of the risk that sexual dysfunction, which is one of the potential side effects of finasteride, may persist even after treatment is discontinued. The action was certified in April 2014, with one of the common issues being whether either drug could cause sexual dysfunction that persists after treatment is stopped. On appeal, Merck argued that the plaintiffs had failed to demonstrate a plausible methodology to prove causation.

Meaning of “Methodology”

The requirement of a methodology to establish common issues has received varied treatment in Canadian courts. The Supreme Court of Canada considered the issue in Pro-Sys Consultants Ltd. v Microsoft , stating that there must be an actual, not just theoretical, methodology through which the common issues may be proven. In its analysis the Court in Miller found that Microsoft appeared to have a “more onerous methodology” requirement because it involved “notoriously complex” indirect purchaser claims, as well as a “massive and diffuse” class.

The Court then considered the Alberta Court of Appeal decision in Andriuk v Merrill Lynch Canada Inc., (which we previously reported on). In that case, the Court decided that, depending on the nature of the case, expert evidence is not necessarily required to establish a methodology to prove causation on a class wide basis, but that some evidence of the existence of a methodology was necessary.

Finally, the Court considered the decisions of the British Columbia Court of Appeal in Stanway v Wyeth Canada Inc. and Charlton v Abbott Industries Ltd. In Stanway, the court was not persuaded that the plaintiff had to establish the methodology by which the causation issue would be determined at the certification stage. However, the evidence in Stanway included a “gold standard" study proving the causal connection, while Miller had no such evidence.

In Charlton, discussed in a previous post on this blog, the Court emphasized that while a class action should not be certified if there is no methodology for addressing the common issue, some evidence of a methodology to prove causation would be sufficient. In that case, the plaintiffs had to prove general harm. There was no consensus among the experts as to the existence of a risk to the class as a whole, nor was there a consensus as to a methodology to prove that risk. This differed from Miller, where general harm was assumed.

In concluding, the Court noted the informational disadvantage of the plaintiffs at the certification hearing stage. In light of this, they suggested that evidence of some of the Bradford-Hill criteria, which are a framework of factors commonly used by scientists to assess proof of causation, may be enough to satisfy a court of the validity of an inference of causation.

Key Takeaway

In Miller the Court found that: an expert opining that a causal link was “biologically plausible”, product labelling changes made by Merck, and evidence of the Bradford-Hill criteria was enough to meet the methodology requirement on common issues. This decision highlights the fact that the requirement for a methodology to prove the common issues is highly dependent on the specific facts of each case.

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Deborah Glendinning

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