Determining sample sizes for combined incident and prevalent cohort studies with and without follow-up

The determination of the sample size is key in the design of a cohort study when requiring a preset statistical power for comparing time to event outcomes of two groups. In complex survival analysis study designs, the time to event data for the two groups can be sampled from a single cohort using a variety of different procedures or, the time to event data can be drawn from a collection of different cohorts. By assuming a unified study design where the observations from various sampling schemes or independent cohort studies are combined, the potential logistical constraints on acquiring a sufficient number of subjects may be mitigated. We derive sample size formulae for data collected from combined incident and prevalent cohort studies with and without follow-up. We show analytically how a combined cohort study requires fewer observations from its individual cohort components relative to studies using data collected solely from a single cohort. We describe how our sample size formulae may be generalized to arbitrary collections of cohort samples and demonstrate, using simulated cohort data, how the proposed combined cohort testing procedure achieves comparable empirical power relative to when the same procedure is applied to data drawn from a single cohort study.