Answer:
The investigator determines how many cases and controls will be in a study based on sample size requirements. Hence, we cannot use these data to estimate incidence as we can in cohort or RCT studies. Since we do not have incidence data, we cannot calculate a relative risk and are, therefore, confined to calculating an odds ratio. The odds ratio approximates the RR when the disease is rare. Since case-control studies are usually done when outcomes are rare, the OR is a good measure of association for this study design. (When the disease is not rare, the OR over-estimates the RR).