In epidemiologic analyses, which of the following statements correctly describes the use of relative risk and odds ratio?

• A. Relative risk can be used in case-control studies, but is less precise than odds ratio.
• B. In any study design, relative risk and odds ratio provide identical measures.
• C. Relative risk is preferred in cohort studies with incidence data; odds ratio approximates RR in case-control studies.
• D. Relative risk is never used; odds ratio is always used.

Answer: (C)

Understanding when to use relative risk versus odds ratio helps you interpret study results correctly. Relative risk compares how likely a disease is to occur in the exposed group versus the unexposed group. You compute it from incidence data, so it fits best in cohort studies or randomized trials where you can follow people over time and see who develops the outcome. It’s a direct measure of risk: how many more (or fewer) people among the exposed develop the disease compared with the unexposed.

Odds ratio, on the other hand, is the ratio of the odds of exposure among those with the disease to the odds of exposure among those without the disease. In practice, this is the natural measure in case-control studies, where you start with cases and controls and don’t observe incidence over time. Because you’re sampling by outcome, you can’t directly compute risks, so the odds ratio is used. When the outcome is rare, the odds ratio closely approximates the relative risk; as the outcome becomes more common, the two diverge.

That’s why the statement that relative risk is preferred in cohort studies with incidence data and that the odds ratio approximates relative risk in case-control studies is the best description. The other options misstate where each measure applies or claim they always provide the same information.