Incidence rate per person-time

Incidence rate divides new events by accumulated person-time and has units such as events per 100 person-years. Cumulative incidence is the proportion of a defined population developing the event over a fixed period; it is dimensionless. With varying follow-up, the rate may be more informative, but still requires clear time at risk. The measures are not interchangeable and answer different questions. Report event definition, inclusion criteria and person-time. Rate and cumulative incidence differ; recurrent events need a clear rule. Uncertainty can remain high with few events despite large person-time. State confidence interval and observation window.

First divide events by total person-years, then multiply by 100. Twelve events over 600 person-years gives 0.02 per person-year or 2 per 100 person-years. This scaling is a more readable unit, not a percentage probability. When follow-up differs, each person contributes time at risk until event, censoring or end of observation. Report event definition, inclusion criteria and person-time. Rate and cumulative incidence differ; recurrent events need a clear rule. Uncertainty can remain high with few events despite large person-time. State confidence interval and observation window.

Events divided by person-time is a descriptive rate and does not automatically require constant hazard. But deriving risk over time from a Poisson or exponential model often assumes a constant rate within groups and intervals. If risk changes with age or duration, stratified rates, time-varying models or survival analysis may be needed. Report follow-up time and event definition. Report event definition, inclusion criteria and person-time. Rate and cumulative incidence differ; recurrent events need a clear rule. Uncertainty can remain high with few events despite large person-time. State confidence interval and observation window.

Reproduce the example calculation: With 12 new events in 600 person-years, rate=12/600=0.02 per person-year, or 2 per 100 person-years. Incidence rate per person-time uses IR = Ereignisse / Personenzeit. In plain terms, the incidence rate relates new events to accumulated time at risk. Unit check: Rate in events/person-time · D event count · Ti time at risk. Requires accurately measured time at risk and consistent event definition; rate need not be constant. Values are comparable only when their definitions, denominators and periods match. Record these details next to the result so the reference group is clear. If the direction is implausible, check data preparation and the calculation basis before discussing biological causes.

A key limit of Incidence rate per person-time is this: Requires accurately measured time at risk and consistent event definition; rate need not be constant. Do not use the metric without its reference; it is useful for Comparing new events when observation time differs across people. The incidence rate relates new events to accumulated time at risk. When comparing populations or experiments, check whether definitions, denominators and observation periods match. If the assumption is implausible, the result shows only what the simplified model calculates from its inputs; it is not a reliable estimate for the target population. State this limitation in the report, and do not infer a causal effect from the formula alone.