Which of the following defines attributable risk (risk difference)?

• A. Subtract the incidence among not exposed from those exposed.
• B. Divide the incidence in exposed by the incidence in the unexposed.
• C. Multiply the incidence in unexposed by the prevalence in the population.
• D. Subtract the incidence in exposed from the incidence in unexposed.

Answer: (C)

Attributable risk (risk difference) reflects how much extra risk the exposed group has compared with the unexposed group. It is calculated as the incidence in the exposed group minus the incidence in the unexposed group (Ie − Iu). This difference tells you how much of the observed risk in the exposed population can be attributed to the exposure itself, assuming other factors stay the same.

Among the options, the subtracting the incidence in the unexposed from the exposed matches this definition (Ie − Iu). Dividing the incidence in the exposed by the unexposed gives a ratio (relative risk), not a risk difference. Multiplying the incidence in the unexposed by the population prevalence isn’t a standard measure of attributable risk, and subtracting the incidence in exposed from that in unexposed would give the opposite sign of the risk difference.