Define population-attributable risk (PAR) and how is it calculated?

• A. PAR% = [Pe*(RR - 1)] / [Pe*(RR - 1) + 1] × 100, where Pe is the prevalence of exposure in the population.
• B. PAR% = [Pe*(RR - 1)] / [RR - 1] × 100
• C. PAR% = Pe × RR × 100
• D. PAR% = [Pe*(RR - 1)] / [Pe*(RR - 1) + 2] × 100

Answer: (A)

Population-attributable risk shows how much of the disease burden in the whole population could be prevented if a specific exposure were eliminated. When expressed as a percent, it uses Pe for the exposure prevalence in the population and RR for the relative risk of disease in exposed versus unexposed. The correct formula is PAR% = [Pe*(RR - 1)] / [Pe*(RR - 1) + 1] × 100. Here, RR - 1 captures the excess risk due to the exposure, and multiplying by Pe gives the population-level excess risk. The denominator adds 1 to reflect the baseline risk in those not exposed, giving the portion of all cases that are attributable to the exposure, and the ×100 converts it to a percentage. Other forms fail because they omit the baseline adjustment or mis-scale the population risk, so they don’t correctly estimate the population-attributable percentage.