What is the standard of review that we must apply on appeal?

"The standard of review that we must apply on appeal is also settled. The trial court is vested with broad discretion to determine whether a class should be certified. 'Our task on appeal is not to determine in the first instance whether the requested class is appropriate but rather whether the trial court has abused its discretion in denying certification.' [Citation.] We 'will not disturb a trial court ruling on class certification which is supported by substantial evidence unless (1) improper criteria were used [citation]; or (2) erroneous legal assumptions were made [citation].' " (Block, supra, 65 Cal.App.4th at pp. 542-543; Linder v. Thrifty Oil Co. (2000) 23 Cal.4th 429, 435-436.)