What does relative risk describe in epidemiology?

• A. The difference in risk between two groups.
• B. The ratio of the risk in an exposed group to that in an unexposed group.
• C. The measure of latency period.
• D. The average risk across all groups.

Answer: (B)

Relative risk describes how much more (or less) likely an outcome is to occur in people who have been exposed to a factor compared with those who have not been exposed. It is calculated by dividing the incidence (or probability) of the outcome in the exposed group by the incidence in the unexposed group. This yields a ratio that tells you the strength of the association: a value greater than 1 means exposure is linked to higher risk, a value less than 1 indicates a protective effect, and a value equal to 1 suggests no difference in risk between groups. In practice, this measure is commonly used in cohort studies where you can track outcomes over time in both exposed and unexposed groups. For example, if 10% of the exposed develop the disease and 2% of the unexposed do, the relative risk would be 0.10 divided by 0.02, which equals 5, indicating a fivefold higher risk with exposure. The other ideas—a simple difference in risk, a measure of latency, or an average risk across groups—do not capture this ratio of risks between groups.