A simple computer program for generating person-time data in cohort studies involving time-related factors.

The use of grouped data methods, such as standardized rate ratios and Poisson regression, for the analysis of cohort studies has a number of attractive features. This approach, however, has not been widely used in the past because of the difficulty of generating person-time data required for the computation of rates, particularly when stratification on time-related factors is involved. This paper presents a simple Statistical Analysis System (SAS) program for the generation of such data in a form that can be read directly by GLIM and used in a Poisson regression analysis. In addition to its simplicity, the program has the advantage of considerable flexibility and involves no restrictions on the number of time-related factors or the number of levels of each factor. Furthermore, it can be modified easily for multiple disease outcomes and for analyses of the latency period, or empirical induction time.