A history of the population attributable fraction and related measures

Since Doll published the first PAF in 1951, it has been a mainstay. Confusion in terminology abounds with regard to these measures. The ability to estimate all of them in case-control studies as well as in cohort studies is not widely appreciated.

This article reviews and comments on the historical development of the population attributable fraction (PAF), the exposed attributable fraction (EAF), the rate difference (ID), the population rate (or incidence) difference (PID), and the caseload difference (CD).

The desire for PAFs to sum to no more than 100% and the interpretation of the complement of a PAF as the proportion of a rate that can be attributed to other causes are shown to stem from the same problem: a failure to recognize the pervasiveness of shared etiologic responsibility among causes. A lack of appreciation that "expected" numbers of cases and deaths are not actually the numbers to be expected when an exposure or intervention appreciably affects person-time denominators for rates, as in the case of smoking and nonnormal body mass, makes many CD estimates inflated. A movement may be gaining momentum to shift away from assuming, often unrealistically, the complete elimination of harmful exposures and toward estimating the effects of realistic interventions. This movement could culminate in a merger of the academic concept of transportability with the applied discipline of risk assessment.

A suggestion is offered to pay more attention to absolute measures such as the rate difference, the population rate difference, and the CD, when the latter can be validly estimated and less attention to proportional measures such as the EAF and PAF.