What is the test for ascertainability in a class action?

"The purpose of the ascertainability requirement is to ensure notice is given to putative class members as to whom a judgment in the class action would be res judicata. [Citation.] Class members are ascertainable when they may be readily identified without unreasonable expense or time by reference to official records. [Citation.] 'In determining whether a class is ascertainable, the trial court examines the class definition, the size of the class and the means of identifying class members.'" (Faulkinbury v. Boyd & Associates, Inc. (2013) 216 Cal.App.4th 220, 240.)