A simple formula for the calculation of sample size in pilot studies

W. Viechtbauer, L. Smits, D. Kotz, L. Budé, M. Spigt, J. Serroyen, R. Crutzen*

*Corresponding author for this work

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One of the goals of a pilot study is to identify unforeseen problems, such as ambiguous inclusion or exclusion criteria or misinterpretations of questionnaire items. Although sample size calculation methods for pilot studies have been proposed, none of them are directed at the goal of problem detection. In this article, we present a simple formula to calculate the sample size needed to be able to identify, with a chosen level of confidence, problems that may arise with a given probability. If a problem exists with 5% probability in a potential study participant, the problem will almost certainly be identified (with 95% confidence) in a pilot study including 59 participants.
Original languageEnglish
Pages (from-to)1375-1379
Number of pages5
JournalJournal of Clinical Epidemiology
Issue number11
Publication statusPublished - Nov 2015


  • Pilot study
  • Sample size
  • Problem detection
  • Rule of three
  • Unforeseen problems

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