In a prospective cohort study, which measure is typically used to quantify the association between exposure and disease?

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

Answer: (A)

In a prospective cohort study the key idea is directly comparing how often the disease occurs in those exposed versus those not exposed. The most informative measure for that is the relative risk, or risk ratio—the incidence in the exposed group divided by the incidence in the unexposed group. This tells you how much more (or less) likely the disease is to develop given the exposure. For example, if 20 per 1,000 exposed develop the disease and 5 per 1,000 unexposed do, the relative risk is 4, meaning the exposed group has four times the risk.

Odds ratio, while useful, compares odds and is most appropriate in case-control designs where incidence isn’t directly observed. Hazard ratio comes from time-to-event analyses (survival models) and accounts for differing follow-up times. Prevalence ratio is used in cross-sectional studies where you measure how many people have the disease at a single point in time.