An efficient Statistical Analysis System computer program for the exact stratification of person-years is described, in order to determine the expected number of deaths or cases of diseases in a cohort for the calculation of standardized mortality ratios or standardized morbidity ratios. A comparison of this program with Macaluso's "Method B" program shows that the speed of the former is considerably faster than that of the latter, when the widths of the categories defined by all of the time factors are identical.