In epidemiology, cohort studies are used to calculate which measure of association?

• A. Relative risk
• B. Odds ratio
• C. Hazard ratio
• D. Prevalence ratio

Answer: (A)

Cohort studies compare the risk of an outcome between groups with and without exposure, so the measure of association is relative risk. This is the ratio of the risk of the outcome in the exposed group to the risk in the unexposed group, showing how much the exposure changes the probability of the outcome. Odds ratio is mainly used in case-control designs or logistic models and can diverge from relative risk when outcomes are common. Hazard ratio comes from time-to-event analyses and accounts for censoring, not the basic cohort design. Prevalence ratio is used in cross-sectional studies. Therefore, relative risk is the best fit for cohort studies.