In Chandler v. City of Dallas (5th Cir. 1993) 2 F.3d 1385 Two employees filed a class action, alleging that a driver safety program which established physical standards for city employees was discriminatory. The federal appellate court held that the class was untenable because:"the effect of a given type of impairment, both on major life activities in general and on a person's ability to perform specific tasks, can vary widely from individual to individual. One person, with impaired vision may simply need to wear glasses, while another may need a guide dog. the prospect of continuing medical advances . . . further supports the need for individualized inquiries in this area. We conclude that class certification and class relief are inappropriate in the instant case." (Id., at p. 1396.) "The party seeking certification as a class representative must establish the existence of an ascertainable class and a well-defined community of interest among the class members. The community of interest requirement embodies three factors: (1) predominant common questions of law or fact; (2) class representatives with claims or defenses typical of the class; (3) class representatives who can adequately represent the class." (Richmond v. Dart Industries, Inc. (1981) 29 Cal. 3d 462, 470 [174 Cal. Rptr. 515, 629 P.2d 23].) "Trial courts have been given great discretion with regard to class certification. . . . In the absence of other error, [an appellate] court will not disturb a trial court ruling on class certification which is supported by substantial evidence unless (1) improper criteria were used . . . or (2) erroneous legal assumptions were made" (Osborne v. Subaru of America, Inc. (1988) 198 Cal. App. 3d 646, 654 [243 Cal. Rptr. 815].)