In a case-control study, why is the odds ratio used, and when does it approximate relative risk?

• A. Because incidence cannot be measured; OR approximates RR when disease is rare.
• B. Because incidence can be measured; OR approximates RR when exposure is rare.
• C. OR and RR are always equal.
• D. Case-control studies measure prevalence.

Answer: (A)

In case-control studies, the measure of association used is the odds ratio because the study design fixes the number of cases and controls, so you can’t measure incidence or risk directly from the data. The odds ratio compares the odds of exposure among cases to the odds of exposure among controls, providing a useful estimate of the association between exposure and disease.

This odds ratio is a good approximation of relative risk when the disease is rare in the source population. When the outcome is rare, the probability of disease in both exposed and unexposed groups is small, so the odds of disease closely track the actual risks, making OR ≈ RR. If the disease is common, the odds ratio can overstate the relative risk, so they diverge.

Also, case-control studies do not measure prevalence or incidence directly, since participants are selected based on disease status rather than followed over time to observe new cases.