What is the standard of proof for a motion for expert certification?

The standard of proof on a motion for certification was at the heart of the appeal in Chadha v. Bayer Inc…. The decision makes clear that at the certification stage, the court cannot engage in any detailed weighing of the evidence but should confine itself to whether there is some basis in the evidence to support the certification requirements. In Chadha, the court denied certification on the basis that there was no evidence that the loss component of liability could be proved on a class-wide basis (and thus that there was no common issue). It was not necessary to establish that there was a compelling method to prove such loss, but it was necessary to provide some basis in fact to think that there was some method to do so. The plaintiffs had failed to provide that basis. This Court reached the opposite conclusion in Pro‑Sys with regard to the commonality of the issues, because there was “an expert methodology that ha[d] been found to have a realistic prospect of establishing loss on a class-wide basis” (para. 140).