title: A simple program to create exact person-time data in cohort analyses.
abstract: BACKGROUND
  Before disease rates can be calculated a tabulation of the length of follow-up for each person in the cohort has to be made. In complicated analyses such tabulations are often stratified by many characteristics, some which show no change with time, such as gender or year of birth, and some which do change with time, such as age or cumulative exposure. Available computer programs often restrict the way these tables can be made, particularly when handling time-dependent variables.
  
  
  METHODS
  This paper presents a simple computer program which calculates the length of follow-up for each person in a study.
  
  
  RESULTS
  Person-time data can be tabulated by a large number of variables using this method. This program is extremely flexible in the way that time-dependent variables can be created, can categorize observations by any unit of person-time, and will run on a range of platforms including a personal computer.
  
  
  CONCLUSIONS
  This method should simplify the task of creating person-time data for analyses of disease rates in epidemiological studies.
authors.affiliation: Department of Epidemiology, School of Public Health, University of North Carolina at Chapel Hill 27599-8050, USA.
authors.forename: J
authors.initials: J
authors.lastname: Wood
authors.forename: D
authors.initials: D
authors.lastname: Richardson
authors.forename: S
authors.initials: S
authors.lastname: Wing
mesh_terms: D015331:Cohort Studies; D004812:Epidemiologic Methods; D005500:Follow-Up Studies; D006801:Humans; D008432:Mathematical Computing; D011381:Programming Languages; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D012984:Software; D012985:Software Design; D013997:Time Factors