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).