personzeit

Personzeit, in epidemiology and public health, is the cumulative amount of time that individuals in a study are observed while at risk of the event of interest. It combines the number of people and the duration of their observation, allowing incidence rates and other time-dependent measures to be expressed relative to time at risk. The standard unit is person-years, though person-months or person-days are also used depending on study length.

Calculation and interpretation: To compute personzeit, sum the time each participant contributes from entry into the at-risk period until the event occurs, loss to follow-up, death, or study end. For example, if 50 participants are followed for four years, the total personzeit is 50 × 4 = 200 person-years. If 20 participants are followed for two years but drop out after six months, and the remaining 80 are followed two years, the total would be (80 × 2) + (20 × 0.5) = 170 person-years. This measure is used to calculate incidence rates as the number of new cases per unit of personzeit (e.g., cases per 1,000 person-years).

Uses and caveats: Personzeit serves as the denominator in incidence rates and other time-to-event statistics. It assumes the risk is distributed over the contributed time and that censoring is non-informative. Care is needed to avoid biases that can arise from misclassifying time, such as immortal time bias, where inappropriate attribution of time to the at-risk period distorts results.