What is the burden on a plaintiff to prove that they have the same claim?

The burden is on the plaintiff, on a certification motion, to adduce "some evidence" to establish that there is "some basis in fact" for the assertion that the claims raise common issues. This standard is far less onerous than the ordinary civil standard of proof on a balance of probabilities. Indeed, it has been described as a low threshold. As Cullity J. stated in Grant v. Canada (Attorney General), [2009] O.J. No. 5232, 81 C.P.C. (6th) 68 (S.C.J.), at para. 21, there must be "some basis in reality" for the assertion of common issues.