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*I am using the pmsampsize function in R to calculate the sample size for a cox proportional hazard model (Pilot study). I need to input the value of meanfup (type="s" option) specifies the average (mean) follow-up time anticipated for individuals in the model development dataset. But still I am not sure how average (mean) follow-up time can be calculated. Any advice would be highly appreciated.

Forexample: Suppose include the information of patients at T0,T3, T6, T9, T12,...., T24 (months).

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If there is at most one event per individual, this meanfup is the average of all observation times: event times for those having the event, the last observation time for those without. For specificity, say that you start with 1000 participants.

First, you need to estimate the distribution of events over time for those 1000 participants, based on your understanding of the subject matter. Then you need to estimate the distribution of last-observation times for those without an event. That will depend on your study design. If you follow all individuals for exactly 24 months, then all those without events (in this example, 1000 minus the number with events within 24 months) will have last-observation times of 24 months. If there is rolling entry into the study, then you might approximate with a uniform distribution of those last-observation times over the total duration of your study.

Then just take the average over all 1000 event/last-observation times.

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