How is population attributable fraction calculated?

• A. AR divided by the prevalence of exposure.
• B. Total incidence minus PAR divided by total incidence.
• C. PAR divided by total incidence in the population.
• D. PAR plus AR.

Answer: (C)

Population attributable fraction represents the proportion of all cases in the population that can be attributed to the exposure. To get it, start with the population attributable risk, the excess incidence in the population due to the exposure, which can be written as Ip − Iu (incidence in the total population minus incidence among the unexposed) or equivalently Pe × (Ie − Iu). The fraction is then obtained by dividing this PAR by the total incidence in the population: PAF = PAR / Ip. So the correct approach is PAR divided by the total incidence in the population. For example, if Ip is 100 cases per 100,000 and PAR is 40 per 100,000, the PAF is 0.40, meaning 40% of cases in the population are attributable to the exposure. The other options don’t express this proportion correctly.