Relative risk is defined as what?

• A. Ratio of incidence rate of exposed individuals to that of controls
• B. Odds ratio
• C. Hazard ratio
• D. Risk difference

Answer: (A)

Relative risk compares how often an outcome occurs in those exposed to a factor versus those who are not. It is the ratio of the incidence in the exposed group to the incidence in the unexposed (controls). This tells you how many times more (or less) likely the outcome is with exposure. For example, if 20 per 1,000 exposed develop the outcome and 10 per 1,000 unexposed do, the relative risk is 2, meaning the exposed have twice the risk. This is a ratio, not a difference in risk. Other measures serve different purposes: a risk difference is a subtraction of risks, the odds ratio compares odds rather than incidence, and the hazard ratio compares instantaneous risk over time. Relative risk uses incidence (or cumulative incidence) data to express how exposure changes risk.