Objectives: To describe a method using confidence intervals (CIs) to estimate the sample size for a pilot randomized trial.
Study design: Using one-sided CIs and the estimated effect size that would be sought in a large trial, we calculated the sample size needed for pilot trials.
Results: Using an 80% one-sided CI, we estimated that a pilot trial should have at least 9% of the sample size of the main planned trial.
Conclusion: Using the estimated effect size difference for the main trial and using a one-sided CI, this allows us to calculate a sample size for a pilot trial, which will make its results more useful than at present.
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