Which statement accurately describes the difference between relative risk and odds ratio and when to use each?

• A. Relative risk compares risk in exposed vs unexposed; odds ratio compares odds of exposure among cases vs controls; use RR in cohort studies; OR approximates RR when the outcome is rare.
• B. Relative risk equals odds ratio in all studies.
• C. OR is used in cohort studies; RR in case-control.
• D. RR is only used when outcome is rare.

Answer: (A)

The main idea is how these two measures reflect association and how study design guides which one to use. Relative risk (risk ratio) directly compares the probability of developing the outcome between exposed and unexposed groups. Since cohort studies start with exposure status and follow people to see who develops the outcome, you can calculate these risks and form the ratio of risks. The odds ratio, by contrast, is based on odds and is the natural summary in case-control designs, where you select participants based on disease status and cannot directly estimate incidence. In that setup, you look at the odds of having been exposed among cases versus controls.

Importantly, when the outcome is rare, the odds of the outcome among exposed and unexposed are very similar to the corresponding risks, so the odds ratio closely approximates the relative risk. But as the outcome becomes more common, the odds ratio can overstate the association compared with the relative risk.

So the best match is: relative risk is used to compare risk in exposed vs unexposed (in cohort studies), and the odds ratio compares odds of exposure among cases vs controls (used in case-control studies); and OR approximates RR when the outcome is rare.