Relative risk and odds ratio: In which study design is relative risk most appropriate?

• A. Case-control studies.
• B. Cross-sectional studies.
• C. Cohort studies.
• D. Ecological studies.

Answer: (C)

Relative risk is the ratio of the probability of developing the outcome among those exposed to the probability among those not exposed, so you need incidence data over time to calculate it directly. Cohort studies are designed to start with people defined by exposure status and then follow them forward to see who develops the outcome, giving you true incidence in both groups and allowing a direct calculation of risk in exposed vs unexposed. In study designs that start with cases and controls based on the outcome, incidence cannot be determined, so you rely on odds ratios instead; the odds ratio approximates relative risk only under certain conditions (e.g., rare disease). Cross-sectional designs measure prevalence at a single point in time, not incidence, so relative risk isn’t directly estimable, and ecological studies give group-level associations rather than individual risk, making RR inappropriate. Hence, the design that best supports relative risk calculation is the cohort study.