worked at least 30 hours per week. This second version of the policy took effect on June 1, 2008. Individual restaurant managers were responsible for determining employee start dates, hours worked, leaves of absence, termination dates, rehire dates, and other basic payroll information that contributed to an employee’s eligibility for earned vacation pay.
In this proposed class action, McCaster alleged that while the first policy was in effect, Darden failed to pay him accrued vacation pay when he left his job at Red Lobster, even though he had earned about 12 vacation hours. Clark, for her part, received all the vacation pay she was owed while the first policy was in effect; she alleged that after June 1, 2008, Darden did not pay her any vacation pay at all when she separated from employment.
In discovery Darden produced five spreadsheets containing statewide payroll information during the relevant time period. As the plaintiffs interpret this data, more than 1,200 employees left Darden’s employ without receiving the pro rata vacation pay they were owed. This interpretation, however, rests entirely on a “declaration” from a paralegal who works at the law firm of one of the plaintiffs’ attorneys. The judge struck the declaration because the paralegal had no personal knowledge of the data, lacked the expertise to interpret it, and the plaintiffs had not designated her as an expert witness. The plaintiffs challenge that ruling on appeal, but they provide no good reason to disturb it. Regardless, the paralegal’s declaration is immaterial to our decision.
The plaintiffs moved for class certification and proposed the following class definition: “All persons separated from hourly employment with [Darden] in Illinois between December 11, 2003, and the conclusion of this action[] who were subject to Darden’s Vacation Policy ... and who did not receive all earned vacation pay benefits.” The district judge rejected this definition because it described an improper fail-safe class. The judge also rejected the plaintiffs’ proposed alternative definition because it failed to meet the requirements of Rule 23.
In the meantime Darden moved for partial summary judgment on Clark’s individual IWPCA claim. The' company argued that no violation of the Act had occurred because during the relevant time period, only full-time employees were eligible for vacation pay and Clark worked part-time. The judge agreed and granted the motion. McCaster settled his individual claim with Darden but reserved the right to appeal the denial of class certification. This appeal followed.
II. Discussion
The plaintiffs seek reversal of the judge’s decision denying class certification. Clark also asks us to reverse the judge’s decision rejecting her IWPCA claim on the merits. The latter issue is quite straightforward, so we’ll take it up first.
A. Clark’s IWPCA Claim
Clark admits that she received all the vacation pay she was owed under Dar-den’s old policy. After June 1, 2008, she was ineligible to receive paid vacation. The company’s new vacation-pay policy, which took effect on that date, covers only full-time employees, defined as those who work at least 30 hours per week. Clark did not qualify because she worked part-time.
Clark concedes the point but argues that if an employer provides paid vacation to its full-time employees on a pro rata length-of-service basis, it may not deny this same benefit to its part-time employees. The district judge rejected this novel interpretation of the IWPCA, and rightly so. It has