What is the test for a class definition in a medical malpractice action?

The court’s concern about a class definition is that it should support a judgment that binds the persons who ought to be bound (a concern about under-inclusiveness) and that it does not bind persons who ought not to be bound (a concern about over-inclusiveness). Viewed from the court’s perspective, an over-inclusive class definition will bind persons who ought not to be bound by judgment or by settlement, be that judgment or settlement favourable or unfavourable. It is to be remembered that until individual trials, if necessary, class members are not direct participants in the action, but they will be bound by the common issues trial and the settlement if approved by the court. The court is concerned about a proper class definition both as a matter of procedural and substantive justice. See Robinson v. Medtronic, [2009] O.J. No. 4366 (S.C.J.) at paras. 133-37.