Table 1

Summary definitions and usual formulas for the attributable fraction

PAFAFE
Population of interestThe whole target population, irrespective of exposure to the risk factorPeople exposed to the risk factor in the target population
Question answeredWhat proportion of the disease burden in a given (target) population is attributable to a given risk factor?What proportion of the disease burden in those exposed in a given (target) population is attributable to a given risk factor?
DefinitionEmbedded Image (Equation 1)Embedded Image (Equation 1′)
Alternative formulasLevin's 1953 formula [3]:
Embedded Image (Equation 2)
Miettinen's 1974 formula [4]:
Embedded Image (Equation 3)		Embedded Image (Equation 2′)

Note that to express PAFs and AFEs as proportions (%), formulas should be further multiplied by 100. The RR for PAF should be based on the same exposure definition as the prevalence, and for AFE on the same exposure definition as for defining the exposed group, and in both cases should be transportable to the target population. Pe: prevalence of exposure in the target population; PeD: prevalence of exposure in diseased people; AFE: attributable fraction in the exposed.