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Caution Regarding the Use of Pilot Studies to Guide Power Calculations for Study Proposals

Helena Chmura Kraemer, PhD; Jim Mintz, PhD; Art Noda, MS; Jared Tinklenberg, MD; Jerome A. Yesavage, MD
Arch Gen Psychiatry. 2006;63(5):484-489. doi:10.1001/archpsyc.63.5.484.
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Clinical researchers often propose (or review committees demand) pilot studies to determine whether a study is worth performing and to guide power calculations. The most likely outcomes are that (1) studies worth performing are aborted and (2) studies that are not aborted are underpowered. There are many excellent reasons for performing pilot studies. The argument herein is not meant to discourage clinical researchers from performing pilot studies (or review committees from requiring them) but simply to caution against their use for the objective of guiding power calculations.

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Figure 1

The power (probability of finding a statistically significant result) using a 1-tailed 5% test when the desired power to detect the effect size defining the threshold of clinical significance (δ*) is 0.5. The upper curve shows results using a priori power calculations. The 3 lower curves show results when the power calculation is based on effect sizes from pilot studies with sample sizes of 20, 50, and 100.

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Figure 2

The probability of aborting a proposal using pilot studies with sample sizes of 20, 50, and 100 when the threshold effect size (δ*) is 0.5.

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Figure 3

The estimated effect size if the study is not aborted, relative to the true effect size using pilot studies with sample sizes of 20, 50, and 100.

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