What is the test for certifying a class?

With regard to the result of finding that the class is overbroad and capable of determination , I am instructed with regard to the judges’ power to amend by the case of Caputo v. Imperial Tobacco Ltd., 2004 CanLII 24753 (ON SC), [2004] O.J. No. 299 (S.C.J.), a decision by Winkler J. (as he then was). In discussing the issue of amending the class definition at paras. 38-45, he states at para. 41: 41 The plaintiffs prevail upon me to amend the class definition to redefine the class in any way necessary to render this action certifiable. In my view, this approach is not what McLachlin C.J. was advocating in Hollick. As I read her reasons, the court may either reject certification where the class is not properly defined or otherwise grant a conditional certification on the basis that the plaintiffs will have to provide an acceptable definition to the court. In some circumstances, it may be appropriate for the court to alter or amend a class definition to be consistent with other findings made on a certification motion. That is not the case here. What the plaintiffs suggest is akin to having the court perform the role of class counsel by making wholesale changes to arrive at a definition that the court itself would accept. That goes beyond a simple exercise of discretion and verges into the prohibited territory of descending "into the arena" with the parties to the motion.