Term: Person-Years / Person Years

Glossary Definition

Last Updated: 2013-10-30


A measurement of observation time per person and is often used as the denominator in incidence rates when, for varying periods, individuals are at risk of developing a disease, using a health service, or dying. Instead of using the number of people at the start of the observation period as the denominator, one can determine for each person the actual time at risk, from the beginning of the study period until the disease is detected, the person is lost to follow-up (i.e., moves out-of-province or dies), or the end of the study period. If a study period is one year and a person is disease free for the entire period then they will have a person-year value of one; while if another person develops a disease six months after the start of the study, they will have a person-year value of 0.5 (Young, 1998).

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