Relative Risk (RR) is used in which type of study to determine how strongly a risk factor is associated with an outcome?

• A. Used in cross-sectional studies to measure incidence rate
• B. Used in case-control studies to estimate odds of exposure
• C. Used in cohort studies to determine how strongly a risk factor is associated with an outcome
• D. Used in randomized trials to compute risk difference

Answer: (C)

Relative risk is about how often the outcome occurs in those exposed compared with those not exposed, using incidence data gathered over time. This requires following people to see who develops the outcome, which is exactly what a cohort study does. By comparing the incidence in the exposed group to the incidence in the unexposed group, you get RR = (incidence in exposed) / (incidence in unexposed). An RR greater than 1 indicates the exposure is associated with a higher risk of the outcome, less than 1 indicates a protective effect, and equal to 1 means no association. Cross-sectional studies measure prevalence at a single point in time and can’t reliably estimate incidence; case-control studies start with outcomes and estimate exposure odds (not incidence), so they use odds ratios; randomized trials can compare risks between groups but the question’s framing points to cohort-design use of RR for the strength of association.