In a cohort study, which measure of association compares the risk of disease between exposed and unexposed groups?

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

Answer: (C)

In a cohort study, the measure that directly compares how often the disease occurs in those exposed versus those unexposed is the relative risk (risk ratio). It is calculated as the incidence (risk) of disease in the exposed group divided by the incidence in the unexposed group. If the result is greater than 1, exposure is associated with a higher risk; if less than 1, exposure is associated with a lower risk.

Hazard ratio, by contrast, comes from time-to-event (survival) analysis and compares hazard rates—the instantaneous risk of developing disease at any given moment, accounting for when events occur and censoring. It is not the same as simply comparing overall risk over the study period, though it can approximate relative risk under certain conditions.

Odds ratio is primarily used in case-control studies, where the actual incidence in a population isn’t measured. Prevalence ratio is used in cross-sectional studies to compare how common a disease is at a single point in time.