How is population attributable risk (PAR) calculated?

• A. Multiply attributable risk by the prevalence of the risk factor in the population.
• B. Subtract incidence among non-exposed from exposed.
• C. Divide population incidence by prevalence.
• D. Add attributable risk and prevalence to obtain PAR.

Answer: (A)

Population attributable risk represents the portion of the total incidence in the population that is due to a specific exposure. It combines how much extra risk the exposure creates (the risk difference between exposed and unexposed) with how common the exposure is in the population. Mathematically, it is the exposure prevalence multiplied by the excess risk from the exposure: PAR = P_exposed × (I_exposed − I_unexposed). This is equivalent to the population incidence minus the incidence that would occur if no one were exposed (I_p − I_0).

Therefore, multiplying the attributable risk (the excess risk from exposure) by the prevalence of the exposure in the population gives the population attributable risk. The other options do not reflect how PAR incorporates both the size of the effect and how widespread the exposure is in the population.