Microcomputer programs for complex epidemiologic procedures. I. Computation of adjusted rates

In epidemiological cohort studies one measure of association based directly on incidence or mortality rates is the excess rate. This measure is defined as the excess in these rates between two groups with different exposure experience. At times the computation of these excess rates separately for each of the categories of a risk factor is crucial, as with these rates we can then determine whether the factor exerts a dose-response effect that may implicate a causal relation. Recently an indirect method of standardization has been reported, which computes the excess rates for a factor that are adjusted for the confounding effects of several other factors. The method involves a complex iterative procedure that cannot be carried out easily without the aid of a computer. The objective of this communication is to make available a microcomputer program that has been written to implement this laborious computation. In this pedagogic note the adjustment method is described and the application of the microcomputer program is illustrated.