What is the test for defining a class?

In Gariepy v. Shell Oil Company (2002), 23 C.P.C. (5th) 360 at para. 47, Nordheimer J. emphasised the need for clarity in the definition of the class: ... a clearly defined class is also required in order that persons will know if they are members of the class so that they may, in turn, decide if they wish to have their rights determined within the class proceeding, or if they would prefer to opt out of the class proceeding and have their rights determined in another fashion, or not at all. It is extremely important, therefore, that the class definition be one where any person can tell, with a minimum of effort, whether he or she is a member of the proposed class. [emphasis added]