How do you compute person-time and why is it used for incidence rate calculations?

• A. Equals number of events
• B. Sums the time each participant is under observation and at risk; used to account for varying follow-up durations
• C. Equals total number of participants
• D. Used only in randomized trials

Answer: (B)

Understanding person-time starts with time at risk. Each participant contributes the amount of time they are observed while they are at risk of the event, from entry into the study until they have the event, are lost to follow-up, or the study ends. By summing these contributions across all participants, you get the total person-time. This lets you compare incidence across groups even when follow-up durations differ, entries occur at different times, or some people drop out.

The incidence rate then uses this total person-time in the denominator: number of new events divided by the total person-time. This standardizes the rate to how long people were actually at risk, producing a rate like events per 100 person-years. For example, if you accumulate 100 person-years of observation and observe 5 new cases, the incidence rate is 5 per 100 person-years. This approach is much more informative than simply counting events or counting participants, because it accounts for varying follow-up times and censoring. It’s widely used in cohort studies and isn’t limited to randomized trials.