Relative Risk is a general term for measures of association calculated from the data in a 2x2 table. Used in cohort studies. In which type of study design is it most commonly used?

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

Answer: (B)

Relative Risk compares how often an outcome occurs in two groups—those exposed to a factor versus those not exposed—by looking at incidence over time. It requires actual risk data, which you get when you follow people and see who develops the outcome; in a 2x2 table this is the ratio of the incidence in the exposed group to the incidence in the unexposed group (RR = [a/(a+b)] / [c/(c+d)]). This direct risk comparison is most straightforward in cohort studies, where you start with disease-free individuals and track them to observe new cases. In case-control studies you start with cases and controls, so you can’t measure incidence directly and rely on the odds ratio instead; cross-sectional studies measure prevalence rather than incidence, and ecological studies use population-level data, making true relative risk less interpretable.