Interpret relative risk in a cohort study and state its null value.

• A. Relative risk compares odds of the outcome between exposed and unexposed.
• B. Relative risk is the hazard ratio.
• C. Relative risk is the ratio of time to event between exposed and unexposed.
• D. Relative risk compares the probability of the outcome in exposed vs unexposed; RR = 1 implies no association; RR > 1 suggests higher risk with exposure.

Answer: (D)

Relative risk measures how the probability of the outcome differs between exposed and unexposed groups in a cohort study. It is the risk in the exposed divided by the risk in the unexposed, and its null value is 1, meaning no difference in risk between the groups. If the relative risk is greater than one, the exposure is associated with higher risk; if it is less than one, the exposure may be protective.

This is distinct from the odds ratio, which compares odds rather than probabilities and is more common in case-control studies (and only approximates relative risk when the outcome is rare). It also differs from the hazard ratio, which comes from survival analysis and reflects the ratio of instantaneous risks over time rather than simple cumulative risk. The idea of a ratio of time to event is tied to time-to-event analysis and is not the relative risk.