OBJECTIVES: Synthesis (e.g. meta-analysis) of incidence rates from observational data requires the total person-time at risk per study. While person-time can be calculated from individual participant data (IPD), for published aggregate data person-time is frequently unreported and requires estimation from the more commonly reported mean/median follow-up duration or Kaplan–Meier curves. There is no standard recommended method to estimate total person-time; here, we compare existing and novel methods.

METHODS: Brief targeted literature searches identified prostate cancer incidence studies fully reporting Kaplan–Meier curves, number at risk tables, median follow-up duration, and total person-time. Methods to estimate person-time were tested, including: multiplication of median follow-up by the number of patients; ’midpoint’ estimation, whereby the reported number of patients at risk in each interval is multiplied by the midpoint between the previous and succeeding timepoints, then summed; and calculation using pseudo-IPD from digitized Kaplan–Meier curves.

RESULTS: Three studies (Mehtälä [2020], Rompay [2019] and Seraphin [2021]) reporting the required data with different follow-up durations (14, 25 and 5 years, respectively) were identified and evaluated. Compared to reported total person-time, estimation from median follow-up yielded an under-estimate (range percentage difference vs reported: −43.23 to −24.07). Midpoint and pseudo-IPD estimation gave closer estimates (range percentage difference vs reported: −8.19 to 2.47 and −7.7 to 0.83, respectively).

CONCLUSIONS: When estimates of person-time are required (e.g. for incidence rate meta-analysis), midpoint and pseudo-IPD methods gave the best estimates. Estimation from median follow-up (the most frequent method in the literature) yielded substantial and consistent under-estimates of person-time, which inflates incidence rate estimates. Accordingly, midpoint or pseudo-IPD methods may be preferred as a unified method where data permits. Where data are limited to median follow-up, further research is required to determine whether estimation of person-time for synthesis of incidence is suitable, and potential biases should be acknowledged and explored via sensitivity analyses.