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Updated: Apr 19, 2026

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Sample Size Requirements and Study Duration for Testing Main Effects and Interactions in Completely Randomized
Barry Kurt Moser1, Susan Halabi1
1Biostatistics and Bioinformatics Department, Box 2717, Duke University Medical Center, Durham, NC 27710.
Abstract:
In this paper we develop the methodology for designing clinical trials with any factorial arrangement when the primary outcome is time to event. We provide a matrix formulation for calculating the sample size and study duration necessary to test any effect with a pre-specified type I error rate and power. Assuming that a time to event follows an exponential distribution, we describe the relationships between the effect size, the power, and the sample size. We present examples for illustration purposes. We provide a simulation study to verify the numerical calculations of the expected number of events and the duration of the trial. The change in the power produced by a reduced number of observations or by accruing no patients to certain factorial combinations is also described.
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