The strength of the population association between an antigen and a disease can be estimated not only by the relative risk value, but also by what variously has been called the population attributable risk and the etiologic fraction. This alternative measure has certain advantages if the association is due to linkage disequilibrium between the antigen allele and a "disease" allele at a closely linked "disease susceptibility" locus. In particular, it can then be used to determine which of many antigens associated with the same disease has the strongest association from a genetical point of view.