Fig. 1

Illustration of the differences in estimates of follow-up using existing and proposed methods. The figure depicts a hypothetical cohort of 100 subjects who were followed and assessed with annual visits for three years. There were 10, 5 and 5 outcome events in the 1st, 2nd and 3rd, respectively. There were 40 dropouts in the 1st year in scenario (A) and in the 3rd year in scenario (B). For simplicity, in this example all events and dropouts occurred on average at the middle of the year. Because the calculation of the true person-time follow-up rate requires the knowledge of the event time for dropouts, we further assumed two situations for the 40 dropouts: (1) none of them became events during the study and (2) 5 of them became events shortly after they dropped out. The Percentage Method (see Eq. (1)) estimates follow-up as the same in both scenarios, since it does not account for person-time in a cohort, and in essence assumes that all dropout occurs at the beginning of the study. Conversely, the Clark Completeness Index (see Eq. (2)) and the Simplified Person-Time Method (see Eq. (5)) both address person-time and provide accurate estimates of the True Person-Time Follow-up Rate (see Eq. (3)). The calculations for each method are shown based on the data from the two scenarios depicted above