A heuristic approach to the formulas for population attributable fraction

Background-As the definitional formula for population attributable fraction is not usually directly usable in applications, separate estimation formul as are required. However, most epidemiology textbooks limit their coverage to Levin's formula, based on the (dichotomous) distribution of the exposure of interest in the population. Few present or explain Miettinen's formula, based on the distribution of the exposure in the cases; and even fewer pre sent the corresponding formulas for situations with more than two levels of exposure. Thus, many health researchers and public health practitioners ar e unaware of, or are not confident in their use of, these formulas, particu larly when they involve several exposure levels, or confounding factors. Methods/Results-A heuristic approach, coupled with pictorial representation s, is offered to help understand and interconnect the structures behind the Levin and Miettinen formulas. The pictorial representation shows how to de al correctly with several exposure levels, and why a commonly used approach is incorrect. Correct and incorrect approaches are also presented for situ ations where estimates must be aggregated over strata of a confounding fact or.