title: Exact stratification of person-years.
abstract: 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.
authors.affiliation: Department of Epidemiology, School of Public Health, University of Alabama, Birmingham 35294.
authors.forename: M
authors.initials: M
authors.lastname: Macaluso
mesh_terms: D003198:Computer Simulation; D004812:Epidemiologic Methods; D005500:Follow-Up Studies; D006801:Humans; D012984:Software; D013997:Time Factors