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A simple formula for the calculation of sample size in pilot studies
Open accessAbstract
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.
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“According to Viechtbauer et al [19], to detect a feasibility issue that has a 10% likelihood of occurrence, a sample size of 30 participants is required to identify the feasibility issue with a 95% CI.”
From Feasibility and Preliminary Efficacy of a “Sit Less” Program Leveraging Fitbit Tracking and Tailored Text Messages in Cardiometabolic Disease: Findings From 2 Parallel Randomized Controlled Trials in Coronary Artery Disease and Type 2 Diabetes (2026) · cites this paper as [19]
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“Viechtbauer et al. (2015) stated that at least 29 participants are sufficient to detect problems with a probability of 0.10 and a 95% confidence interval.”
From Feasibility of a Software agent providing a brief Intervention for Self-help to Uplift psychological wellbeing (“SISU”). A single-group pretest-posttest trial investigating the potential of SISU to act as therapeutic agent (2021) · cites this paper as Viechtbauer et al., 2015
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Cites
A tutorial on pilot studies: the what, why and how
Thabane, L., Ma, J., Chu, R., Cheng, J., Ismaila, A., Rios, L. P., Robson, R., Thabane, M., Giangregorio, L., & Goldsmith, C. H. (2010) · BMC Medical Research Methodology · doi:10.1186/1471-2288-10-1 · Cited by 3,384
Sample size of 12 per group rule of thumb for a pilot study
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Considerations in determining sample size for pilot studies
Hertzog, M. A. (2008) · Research in Nursing & Health · doi:10.1002/nur.20247 · Cited by 2,337
What is a pilot or feasibility study? A review of current practice and editorial policy
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If nothing goes wrong, is everything all right? Interpreting zero numerators
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Initial Scale Development: Sample Size for Pilot Studies
Johanson, G. A., & Brooks, G. P. (2010) · Educational and Psychological Measurement · doi:10.1177/0013164409355692 · Cited by 1,012 · cited 2×
“These considerations have led to various guidelines for choosing an appropriate sample size for a pilot study, such as 12 participants per group [3], values in the range of 10 to 40 participants per group depending on the parameter of interest [4,5], at least 9% of the main trial’s sample size [6], or at least 50 part…”
BackgroundOn the use of a pilot sample for sample size determination
Browne, R. H. (1995) · Statistics in Medicine · doi:10.1002/sim.4780141709 · Cited by 1,007
. Caution regarding the use of pilot studies to guide power calculations for study proposals.
Kraemer, H. C., Mintz, J., Noda, A., Tinklenberg, J., & Yesavage, J. A. (2006) · Archives of General Psychiatry · doi:10.1001/archpsyc.63.5.484 · Cited by 702 · cited 2×
“smaller scale can help to identify unforeseen problems that could compromise the quality or flow of the study [2].”
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Metadata
- Authors
- Viechtbauer, W., Smits, L., Kotz, D., Budé, L., Spigt, M., Serroyen, J., & Crutzen, R.
- Affiliations
- Maastricht University (NL)
- Corresponding author
- Crutzen, Rik
- Venue
- Journal of Clinical Epidemiology
- Publisher
- Elsevier BV
- Published
- 2015-11
- Volume
- 68
- Issue
- 11
- Pages
- 1375-1379
- ISSN
- 0895-4356 (ISSN-L)
- OpenAlex
- W1763526952
- Cited by
- 612
- References
- 15
- Citations by year
- 2026: 56 · 2025: 91 · 2024: 93 · 2023: 82 · 2022: 77 · 2021: 69 · 2020: 50 · 2019: 39 · 2018: 31 · 2017: 16 · 2016: 8
- FWCI
- 16.32
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Locations
- Journal of Clinical Epidemiology (publishedVersion)
- PubMed (publishedVersion)
- Research Publications (Maastricht University) (publishedVersion, other-oa)
- Lirias (submittedVersion)
- Research Publications (Maastricht University) (submittedVersion, other-oa)
- Topics
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Epidemiology
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Clinical Trials
- Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › Meta-analysis and systematic reviews
- Keywords
- sample size determination, pilot study, problem identification, inclusion and exclusion criteria, statistical power
- MeSH terms
- Humans, Pilot Projects, Sample Size, Mathematical Concepts
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