In a cohort study, which measure quantifies how much more likely the disease is among the exposed compared with the unexposed?

• A. Attack Rate
• B. Relative Risk
• C. Prevalence Ratio
• D. Odds Ratio

Answer: (B)

In a cohort study, you follow groups over time to see who develops the disease, so the measure you use is the relative risk. This directly compares the risk of getting the disease in the exposed group to the risk in the unexposed group. If the relative risk is greater than 1, the exposure is associated with a higher risk of disease; if it’s less than 1, the exposure may be protective; if it equals 1, there’s no association. Relative risk is computed from incidence (or risk) in each group and is the most straightforward way to quantify how much more likely the disease is with exposure.

Attack rate describes the risk during an outbreak and is a form of cumulative incidence used in that specific context, but it’s not the standard measure for assessing exposure-outcome risk across a cohort. Prevalence ratio compares existing cases rather than new cases (incidence), which shifts the focus away from risk over time. Odds ratio compares odds rather than probabilities and is especially common in case-control studies where incidence cannot be directly measured; in cohorts you can use relative risk directly, and the odds ratio can overestimate risk when the disease is common.