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In Reply.听
鈥擶e agree with Levy that the selection of suitable controls is the most difficult and most important aspect in the design of a case-control study. His main objection to our study design is that we did not match our cases and controls with regard to contact lens wear schedule. This was intentional, since we were not primarily interested in the lens material itself as a risk factor for ulcerative keratitis in isolation from the interaction of patients with contact lens products. Rather, we were interested primarily in the more clinically relevant question of the risk of ulcerative keratitis among users of different contact lens types.In a case-control study, cases and controls are compared with respect to the attributes or exposures thought to be related to the disease under consideration. Controls should ideally be matched to cases in all respects except for the risk factor(s) of interest. Any