Which statement defines relative risk (risk ratio)?

• A. Divide the incidence in the exposed by the incidence in the unexposed.
• B. Subtract the incidence in the unexposed from the exposed.
• C. Ratio of incidence in the unexposed to the exposed.
• D. Difference between the incidence in the exposed and the unexposed.

Answer: (A)

Relative risk compares how often an outcome occurs in the exposed group versus the unexposed group, using a ratio of their incidences. It is Ie divided by Iu, where Ie is the incidence in those with the exposure and Iu is the incidence in those without. This makes the measure unitless and easy to interpret: a value greater than 1 means higher risk with exposure, less than 1 means lower risk, and equal to 1 means no association. For example, if 20% of exposed individuals experience the outcome and 10% of unexposed do, the relative risk is 0.20/0.10 = 2, indicating the exposure is associated with twice the risk.

The other formulations describe a risk difference (subtracting incidences) or an inverted or non-ratio comparison, which do not reflect the relative-risk concept.