Which statement correctly describes relative risk and odds ratio?

• A. OR is always equal to RR
• B. OR approximates RR when outcomes are rare
• C. RR compares odds
• D. RR is used only in case-control studies

Answer: (B)

Relative risk and odds ratio are different ways to measure the association between exposure and outcome. Relative risk compares probabilities (risk in the exposed group divided by risk in the unexposed group), while odds ratio compares odds (the ratio of the probability of the event to the probability of no event, in each group). When the outcome is rare, the probability of the event is small, so odds ≈ probability. In that case, the odds ratio closely approximates the relative risk, which is why this statement is correct. The other statements mix up what RR and OR measure and when each is used—for example, RR is not a measure of odds, and RR is not limited to case-control studies (that design often uses OR because risks cannot be directly estimated). When the outcome is common, OR can overstate the strength of association compared with RR.