What is the test for "common issues" in a class action?

A more difficult question is whether "the claims ... of the class members raise common issues", as required by s. 5(1) of the Class Proceedings Act, 1992. As I wrote in Western Canadian Shopping Centres, the underlying question is "whether allowing the suit to proceed as a representative one will avoid duplication of fact‑finding or legal analysis". Thus an issue will be common "only where its resolution is necessary to the resolution of each class member's claim" (para. 39). Further, an issue will not be "common" in the requisite sense unless the issue is a "substantial ... ingredient" of each of the class members' claims. Rumley v. British Columbia, 2001 SCC 69: