Errors in the Calculation of the Population Attributable Fraction.

One of the common errors in the calculation of the population attributable fraction (PAF) is the use of an adjusted risk ratio in the Levin formula. In this article, we discuss the errors visually using wireframes by varying the standardized mortality ratio (SMR) and associational risk ratio (aRR) when the prevalence of exposure is fixed. When SMR > 1 and SMR > aRR, the absolute bias is positive, and its magnitude increases as the difference between SMR and aRR increases. By contrast, when aRR > SMR > 1, the absolute bias is negative and its magnitude is relatively small. Moreover, when SMR > aRR, the relative bias is larger than 1, whereas when SMR < aRR, the relative bias is smaller than 1. Although the target population of the PAF is the total population, the target of causation of the PAF is not the total population but the exposed group.