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

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

J Clin Epidemiol. 2015 Jun 6. pii: S0895-4356(15)00303-0. doi: 10.1016/j.jclinepi.2015.04.014. [Epub ahead of print]

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.

Online sample size calculator for pilot studies: www.pilotsamplesize.com

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