In a cohort study, which comparison best reflects how risk is assessed?

• A. Mortality rate by exposure status
• B. Prevalence of disease at a single time point
• C. Incidence of disease between exposed and unexposed groups
• D. Change in exposure status over time without outcome data

Answer: (C)

In a cohort study, risk is best reflected by looking at how often new cases of the disease occur in groups defined by exposure over time. The study design follows people who start disease-free, compares those who are exposed to those who are not, and measures the incidence of the disease as new cases arise. This direct assessment of incident cases allows calculation of risk or relative risk between the two groups, which is the essence of how risk is evaluated in cohorts.

Prevalence at a single time point isn’t about new cases over time—it's a snapshot that can be influenced by both incidence and duration of disease, so it doesn’t capture risk the way a cohort does. Mortality rate by exposure status focuses on death outcomes and, while possible in a cohort, it shifts the endpoint away from disease incidence to a different outcome. Change in exposure status over time without outcome data provides no information on how many people develop the disease.