How is the population attributable fraction (PAF) defined?

• A. PAR divided by total incidence of that disease in the population.
• B. AR multiplied by the prevalence of exposure.
• C. PAR plus AR.
• D. The prevalence of exposure times RR.

Answer: (A)

PAF measures how much of the disease burden in a population is due to a specific exposure and how much could be prevented if that exposure were eliminated. It is defined as the population attributable risk divided by the total incidence in the population. Here, the population attributable risk equals the excess incidence in the population caused by the exposure (I minus the incidence that would occur if no one were exposed, I0). So PAF = (I − I0) / I. This is exactly the ratio of the excess cases to all cases, which is why it matches the stated definition.

For context, if the overall disease incidence is 10 per 1000 and removing the exposure would reduce it to 6 per 1000, the PAR is 4 per 1000, and the PAF is 4/10 = 0.4 (40%).

The other options don’t match this definition: they mix or misrepresent the components (for example, multiplying risk differences by exposure prevalence, or adding quantities, or using a product of prevalence and relative risk) in ways that do not yield the fraction of population cases attributable to the exposure.