Proportion of Disease Caused or Prevented by a Given Exposure, Trait or Intervention1

The structures of two epidemiologic parameters are explored. One, the “etiologic fraction, ” relates to markers of increased risk, and it is the proportion of disease attributable to the marker and/or to factors associated with it. The other, the “prevented fraction, ” is the equivalent of this for a marker of reduced risk. It is shown that both parameters depend—in different ways—-on the frequency of the marker among cases of the disease, and on the “standardized morbidity ratio” for those with the marker. Point estimation of these parameters is often straight-forward, particularly in case-control studies.