# Metrics: Population Attributable Fraction (PAF)

### Definition

The contribution of a risk factor to a disease or a death is quantified using the population attributable fraction (PAF). PAF is the proportional reduction in population disease or mortality that would occur if exposure to a risk factor were reduced to an alternative ideal exposure scenario (eg. no tobacco use). Many diseases are caused by multiple risk factors, and individual risk factors may interact in their impact on overall risk of disease. As a result, PAFs for individual risk factors often overlap and add up to more than 100 percent.

### Calculating PAF

• Pi = proportion of population at exposure level i, current exposure
• P'i = proportion of population at exposure level i, counterfactual or ideal level of exposure
• RR = the relative risk at exposure level i
• n = the number of exposure levels

For risk factors with continuous rather than discrete exposure levels there is an analogous formula for PAF involving integration of the exposure level distribution.