This paper describes methods for the exact assignment of person-years of follow-up to strata of time-dependent factors. It reviews methods applicable to variables that directly measure the passing of time and that increase in step with follow-up (monotonic time factors), and it extends their use to nonmonotonic time factors. When the same categorization scheme is used for all time factors (for example, 5-year categories), the follow-up period consists of segments whose length and position in time are predictable. A new exact method presented here uses this property to improve the efficiency of computations. The new method is generally faster than a method previously described. Finally, the paper outlines a method of exact person-year stratification that is applicable to all time-dependent factors. (Epidemiology 1992;3:441-448)
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