Exposure opportunity in case-control studies.

Efficient control of confounding is well recognized as a legitimate motive for restriction, matching, and conditional analysis in case-control studies. Some investigators, however, have advocated use of the same techniques for another purpose: to achieve a balance of "opportunity" or "potential" for past exposure between cases and controls. This paper shows that disparities of exposure opportunity between cases and controls exert no bias on estimates of the incidence rate ratio. The precision of rate ratio estimates is lessened when case-control studies are reduced in size by the exclusion of people with no opportunity for exposure. Matching and conditional analysis with respect to indicators of exposure opportunity also reduce precision without enhancing validity. Such indicators, which are correlates of exposure but not determinants of disease, are not confounders; therefore, they do not need to be controlled in the design or analysis of case-control studies.