Original articleA history of the population attributable fraction and related measures
Introduction
The population attributable fraction (PAF) answers the following question: suppose every member of a population who was not in the most favorable level of an exposure or some other condition or event with regard to an adverse outcome had been shifted into that level. By what proportion would the entire population's rate, hazard, risk, prevalence, or caseload have been reduced?
The brief history to follow of the PAF and related measures begins in the early 1950s and ends, not altogether arbitrarily, at the close of the 1980s. The primary focus is on matters of interpretation, with some attention paid to the influence of rather basic features of study design and data analysis. Some statistical issues, such as those pertaining to sampling error and covariate adjustment, are not addressed.
Section snippets
The first PAF
In 1951, Doll [1] estimated what appears to be the first published PAF in the epidemiologic literature. He used the cases from his preliminary (1948–1949) case-control study with Hill [2] to form the numerators for lung cancer incidence rates in Greater London. To obtain the denominators, he apportioned census figures by the smoking distribution in the study's control group. Within each age stratum, he multiplied the total person-time at risk by the rate among the nonsmokers “to estimate the
1954 to 1959
Gefeller wrote that the PAF “fell into oblivion” [15] after 1953. Indeed, the citation to Levin's 1953 article [13] in a 1958 review of statistical methods in cancer research [16] was for another contribution entirely.
Another sign of early indifference to the PAF came in 1954, when the authors of two articles [7], [8] used several case-control studies to break down an external target population's overall lung cancer rate by smoking as Cornfield [6] and Doll [3] had done. Although the
The ID, EAF, PID, and PAF in an early textbook
In their 1960 textbook, Epidemiologic Methods [17], MacMahon, Pugh, and Ipsen described all four measures of primary interest here. They called the ID the “attributable risk” and the PID the “attributable community risk” but gave no names to the EAF or the PAF. The authors wrote, “The concept of attributable community risk is useful in that it provides an estimate of the maximum reduction of a particular disease rate that might be expected if the specified exposure were removed” [17].
With lung
Lilienfeld's praise for the PAF
The PAF received a big boost in 1973 when Lilienfeld, an early adopter, praised its utility in the publication of the American Public Health Association's first Wade Hampton Frost Lecture [26], which he had given the year before. He stated that the PAF is “important to the public health administrator,” but did not elaborate on that use of the measure. His main focus was on what he saw as its implications for setting research priorities.
As Lilienfeld described it, the higher the PAF for a given
An early PAF generalization in theory
In articles in 1975 [33] and 1976 [34], Walter continued with the assumption that attributable fraction of incidence rates are equivalent with attributable fractions of incident caseloads. In the longer article [34], he examined all the measures under consideration here except the PID. He called the ID “Berkson's simple difference” and, following Lilienfeld, called the PAF “the attributable risk” and the EAF “the attributable risk among the exposed.”
Walter [34] stated that exposure-specific
More PAF theory and generalized PAF practice
In 1986, Rothman [44] criticized Doll and Peto [37] for ignoring shared causal responsibility and thereby forcing the sum of their PAFs for causes of cancer not to exceed 100%. In an update, Peto raised the ceiling to 200% [45].
The 1980s also saw other researchers begin to follow in the footsteps of Ouellet et al. [35] and estimate attributable proportions for shifts in exposure distributions more realistic than moving everyone to the exposure level with the lowest rate. In one article,
Comment
Although by the 1990s, the basic elements of the points emphasized here were in place in the literature on the PAF and related measures, that literature has continued to blossom. Particularly, notable contributions have been made by Greenland [50], [51], [52], [53], [54], [55], [56], Rockhill et al. [57], [58].
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