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ARFAR: a person years at risk program.

Cohort studies are often used to compare the health experience of groups of people. Several methods of analysis exist, such as the use of standardised or proportional mortality rates' and the proportional hazards model.2 The calculation of a standardised mortality ratio (SMR) needs to take into account the effect of confounding factors, such as age and sex, which may affect mortality rates. The use of person years at risk to calculate an SMR is a means of controlling for these factors.3 Person years at risk are the number of years that the members of the study population contribute to the study. The person years may be divided into categories that contain only contributions from comparable individuals, thus controlling for the confounding effect. For example, since mortality rates vary with age the total person years at risk may be split into, say, five year age categories from age 20 to 70. Mortality rates applicable to each age group may then be used to obtain an expected number of deaths in each age group and hence for all age groups. These rates might be taken from national figures, although this would not control for any "healthy worker" effect.4 The number of expected deaths may then be compared with the deaths observed in the study population. Similarly, it is possible to control for changes in mortality rates over time for a given age, by splitting the person years at risk according to calendar year. In the figure a subject enters the study at point X and leaves at point Y, contributing at different times person years at risk to five age/calendar year categories. The appropriate age and calendar year specific death rates may be used to derive an expected number of deaths. Further divisions may also be made to allow for sex, race, or length of time in the study. The calculation of person years at risk is a tedious task. Even splitting by as few as three factors can generate several hundred separate categories to use for the calculations. Several computer programs exist which enable the person years at risk to be calculated, among them MYCL, Man Years Computing Language.5 These programs allow the user to specify how