Personenjahre

Personenjahre is a unit of time used in epidemiology and public health to quantify the amount of observation time during which study participants are at risk. One Personenjahr corresponds to one person being observed for one year. For example, ten participants observed for six months each contribute five Personenjahre of observation time.

Computation and interpretation: To determine the total Personenjahre in a study, sum the follow-up time for all participants until an event occurs, they are lost to follow-up, die, or the study ends. The resulting total is the number of Personenjahre. Incidence rates are often expressed per Personenjahre: incidence rate = number of new cases divided by total Personenjahre. For instance, if five new cases occur over 200 Personenjahre, the incidence rate is 0.025 per Personenjahr, or 2.5 per 100 Personenjahre.

Advantages and limitations: Using Personenjahre allows comparison of risk when participants have different lengths of follow-up and when there is censoring. It accommodates staggered entry and varying observation times. However, it assumes a relatively stable hazard over the observation period and depends on accurate measurement of follow-up time. It may be less informative when the risk changes rapidly over time.

Applications and relation to other measures: Personenjahre are widely used in cohort studies, clinical trials, and pharmacoepidemiology to compute incidence rates and to relate observed events to time at risk. They are distinct from cumulative incidence, which summarizes risk over a fixed period without directly incorporating exact time at risk.