Risk and rate advancement periods as measures of exposure impact on the occurrence of chronic diseases.

The predominant epidemiologic measures of risk factor impact have been various concepts of the "attributable fraction." For many chronic diseases, however, risk factors merely advance the occurrence of disease, and traditional concepts of the attributable fraction do not convey information on the time dimension of premature disease occurrence. Measures of years of disease-free life lost have been proposed to reflect this time dimension but are not always estimable without special assumptions. In this paper, we examine two alternative measures, the risk and rate advancement periods, which are the time periods by which the risk or rate of disease is advanced among exposed subjects conditional on disease-free survival to some baseline age. These measures are applicable for risk factors that promote progression of chronic diseases whose rates increase with age. Point and interval estimates of these measures are easily derived from the output of standard modeling analyses. The measures are illustrated with examples from the literature. The uses and limitations of risk and rate advancement periods are critically discussed.