In a cohort study, what measure is typically used to express the association between exposure and disease?

• A. Relative risk
• B. Case-control method
• C. Odds ratio
• D. Cross-sectional

Answer: (A)

Cohort studies track people over time to see who develops disease based on their exposure, which lets you compare how often the disease occurs in each group. The best way to express that difference is the relative risk, also called the risk ratio. It’s the incidence of disease among the exposed divided by the incidence among the unexposed. For example, if 30 out of 200 exposed individuals develop the disease and 10 out of 200 unexposed do, the relative risk is (30/200) ÷ (10/200) = 3. This means the exposed group has three times the risk of developing the disease compared to the unexposed group.

Other designs fit different measures: in case-control studies you can’t directly measure incidence, so you use the odds ratio to express association; in cross-sectional studies you measure prevalence at one point in time and may report prevalence ratios or odds ratios rather than relative risk.