personyears

Person-years is a unit used to quantify the total amount of time that individuals in a study are under observation and at risk. One person-year represents one person being observed for one year. It is commonly used in epidemiology, demography, and clinical research to standardize follow-up time across study subjects.

Calculation involves summing the time each participant contributes from entry into the study to the occurrence of an event, loss to follow-up, death, or the end of the study. Time is typically measured in years, and partial years are counted proportionally (for example, 6 months of follow-up contributes 0.5 person-years). Example: 100 participants followed for one year contribute 100 person-years; 50 followed for 2 years and 50 for 1 year contribute 150 person-years.

Uses include the computation of incidence rates, which are often expressed per 1,000 or 100,000 person-years. The incidence rate equals the number of new events divided by total person-years, allowing comparison across groups with different follow-up times and aiding in the handling of censored data in survival analyses.

Interpretation and limitations: Person-years measure observation time, not risk itself. They reflect study design, censoring, and reporting practices and should be interpreted in context with the underlying hazard and population. They do not provide a direct estimate of risk without additional information about the event count and timing.