Sample size calculations for pilot cluster-randomised controlled trial: a review 2010-2020

Rebecca M. Simpson; Jen Lewis; Michael J. Campbell; Lauren Desoysa; Peter J. Dodd; Dan Green; Bright C. Offorha; Ines Rombach; Stephen J. Walters; Steven Julious

doi:10.1186/s40814-026-01792-z

Sample size calculations for pilot cluster-randomised controlled trial: a review 2010-2020

Rebecca M. Simpson
, Jen Lewis
, Michael J. Campbell
, Lauren Desoysa
, Peter J. Dodd
, Dan Green
, Bright C. Offorha
, Ines Rombach
, Stephen J. Walters
, Steven Julious

University of Sheffield
University of Manchester

Research output: Contribution to journal › Review article › peer-review

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Abstract

BACKGROUND: There is no recommended guidance on appropriate sample sizes for pilot cluster-randomised controlled trials (cRCTs). Pilot trials should not aim to demonstrate efficacy, and achieving power should not be used to justify the sample size. However, the CONSORT extension for pilot trials states that some justification for their sample size should be given. We conducted a review to understand the choices and justifications of pilot cRCT sample sizes and their trends over time, and to explore apparent changes following the publication of CONSORT extensions for cluster trials and for pilot and feasibility trials.

METHODS: We searched PubMed and Web of Science for pilot cRCTs. The search took place on 01/10/2020 and was restricted to papers published on or after 01/01/2010. Identification of papers was based around a search for the terms 'pilot' or 'feasibility' in the title and abstract/topic. The primary interest in the review was in the planned sample size in terms of clusters per-arm. We also examined participants per-arm and enrolled sample size. Analyses were descriptive or displayed graphically.

RESULTS: Our search returned 3090 records. After removing exact duplicates, aggregating records into unique studies and excluding ineligible studies, we identified 170 pilot or feasibility cRCTs. The median sample size was four clusters per arm. Stratification showed this to be fairly consistent, regardless of the justification given, whether a formal analysis was planned, whether the intention was to estimate the Intra-Cluster Correlation, cluster type, general medical area, funding type, and over time.

CONCLUSION: Average sample sizes for cRCTs have remained strikingly constant over the period 2010-2020 and across several key features of studies; they do not appear to be meaningfully impacted by the stated study aims or sample size justifications. This is despite the fact that the reported main aims for pilot cRCTs, and justifications for their sample sizes, do appear to have changed during this time. Given that aims and justifications appear to have changed, but typical sample sizes have not, it is possible that some researchers choose sample size for pilot cRCTs primarily out of convenience or practical reasons, whilst stating other justifications.

Original language	English
Article number	54
Number of pages	15
Journal	Pilot and Feasibility Studies
Volume	12
Issue number	1
Early online date	16 Mar 2026
DOIs	https://doi.org/10.1186/s40814-026-01792-z
Publication status	Published - 23 Apr 2026

Bibliographical note

Copyright ©The Author(s) 2026. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/

Keywords

Cluster trials
Pilot studies
Sample sizes

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s40814-026-01792-z_reference
Copyright ©The Author(s) 2026. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/
Accepted author manuscript, 1.62 MBLicence: CC BY 4.0

Cite this

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title = "Sample size calculations for pilot cluster-randomised controlled trial: a review 2010-2020",

abstract = "BACKGROUND: There is no recommended guidance on appropriate sample sizes for pilot cluster-randomised controlled trials (cRCTs). Pilot trials should not aim to demonstrate efficacy, and achieving power should not be used to justify the sample size. However, the CONSORT extension for pilot trials states that some justification for their sample size should be given. We conducted a review to understand the choices and justifications of pilot cRCT sample sizes and their trends over time, and to explore apparent changes following the publication of CONSORT extensions for cluster trials and for pilot and feasibility trials.METHODS: We searched PubMed and Web of Science for pilot cRCTs. The search took place on 01/10/2020 and was restricted to papers published on or after 01/01/2010. Identification of papers was based around a search for the terms 'pilot' or 'feasibility' in the title and abstract/topic. The primary interest in the review was in the planned sample size in terms of clusters per-arm. We also examined participants per-arm and enrolled sample size. Analyses were descriptive or displayed graphically.RESULTS: Our search returned 3090 records. After removing exact duplicates, aggregating records into unique studies and excluding ineligible studies, we identified 170 pilot or feasibility cRCTs. The median sample size was four clusters per arm. Stratification showed this to be fairly consistent, regardless of the justification given, whether a formal analysis was planned, whether the intention was to estimate the Intra-Cluster Correlation, cluster type, general medical area, funding type, and over time.CONCLUSION: Average sample sizes for cRCTs have remained strikingly constant over the period 2010-2020 and across several key features of studies; they do not appear to be meaningfully impacted by the stated study aims or sample size justifications. This is despite the fact that the reported main aims for pilot cRCTs, and justifications for their sample sizes, do appear to have changed during this time. Given that aims and justifications appear to have changed, but typical sample sizes have not, it is possible that some researchers choose sample size for pilot cRCTs primarily out of convenience or practical reasons, whilst stating other justifications.",

keywords = "Cluster trials, Pilot studies, Sample sizes",

author = "Simpson, \{Rebecca M.\} and Jen Lewis and Campbell, \{Michael J.\} and Lauren Desoysa and Dodd, \{Peter J.\} and Dan Green and Offorha, \{Bright C.\} and Ines Rombach and Walters, \{Stephen J.\} and Steven Julious",

note = "Copyright {\textcopyright}The Author(s) 2026. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/",

year = "2026",

month = apr,

day = "23",

doi = "10.1186/s40814-026-01792-z",

language = "English",

volume = "12",

journal = "Pilot and Feasibility Studies",

issn = "2055-5784",

publisher = "BioMed Central",

number = "1",

}

TY - JOUR

T1 - Sample size calculations for pilot cluster-randomised controlled trial: a review 2010-2020

AU - Simpson, Rebecca M.

AU - Lewis, Jen

AU - Campbell, Michael J.

AU - Desoysa, Lauren

AU - Dodd, Peter J.

AU - Green, Dan

AU - Offorha, Bright C.

AU - Rombach, Ines

AU - Walters, Stephen J.

AU - Julious, Steven

N1 - Copyright ©The Author(s) 2026. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/

PY - 2026/4/23

Y1 - 2026/4/23

N2 - BACKGROUND: There is no recommended guidance on appropriate sample sizes for pilot cluster-randomised controlled trials (cRCTs). Pilot trials should not aim to demonstrate efficacy, and achieving power should not be used to justify the sample size. However, the CONSORT extension for pilot trials states that some justification for their sample size should be given. We conducted a review to understand the choices and justifications of pilot cRCT sample sizes and their trends over time, and to explore apparent changes following the publication of CONSORT extensions for cluster trials and for pilot and feasibility trials.METHODS: We searched PubMed and Web of Science for pilot cRCTs. The search took place on 01/10/2020 and was restricted to papers published on or after 01/01/2010. Identification of papers was based around a search for the terms 'pilot' or 'feasibility' in the title and abstract/topic. The primary interest in the review was in the planned sample size in terms of clusters per-arm. We also examined participants per-arm and enrolled sample size. Analyses were descriptive or displayed graphically.RESULTS: Our search returned 3090 records. After removing exact duplicates, aggregating records into unique studies and excluding ineligible studies, we identified 170 pilot or feasibility cRCTs. The median sample size was four clusters per arm. Stratification showed this to be fairly consistent, regardless of the justification given, whether a formal analysis was planned, whether the intention was to estimate the Intra-Cluster Correlation, cluster type, general medical area, funding type, and over time.CONCLUSION: Average sample sizes for cRCTs have remained strikingly constant over the period 2010-2020 and across several key features of studies; they do not appear to be meaningfully impacted by the stated study aims or sample size justifications. This is despite the fact that the reported main aims for pilot cRCTs, and justifications for their sample sizes, do appear to have changed during this time. Given that aims and justifications appear to have changed, but typical sample sizes have not, it is possible that some researchers choose sample size for pilot cRCTs primarily out of convenience or practical reasons, whilst stating other justifications.

AB - BACKGROUND: There is no recommended guidance on appropriate sample sizes for pilot cluster-randomised controlled trials (cRCTs). Pilot trials should not aim to demonstrate efficacy, and achieving power should not be used to justify the sample size. However, the CONSORT extension for pilot trials states that some justification for their sample size should be given. We conducted a review to understand the choices and justifications of pilot cRCT sample sizes and their trends over time, and to explore apparent changes following the publication of CONSORT extensions for cluster trials and for pilot and feasibility trials.METHODS: We searched PubMed and Web of Science for pilot cRCTs. The search took place on 01/10/2020 and was restricted to papers published on or after 01/01/2010. Identification of papers was based around a search for the terms 'pilot' or 'feasibility' in the title and abstract/topic. The primary interest in the review was in the planned sample size in terms of clusters per-arm. We also examined participants per-arm and enrolled sample size. Analyses were descriptive or displayed graphically.RESULTS: Our search returned 3090 records. After removing exact duplicates, aggregating records into unique studies and excluding ineligible studies, we identified 170 pilot or feasibility cRCTs. The median sample size was four clusters per arm. Stratification showed this to be fairly consistent, regardless of the justification given, whether a formal analysis was planned, whether the intention was to estimate the Intra-Cluster Correlation, cluster type, general medical area, funding type, and over time.CONCLUSION: Average sample sizes for cRCTs have remained strikingly constant over the period 2010-2020 and across several key features of studies; they do not appear to be meaningfully impacted by the stated study aims or sample size justifications. This is despite the fact that the reported main aims for pilot cRCTs, and justifications for their sample sizes, do appear to have changed during this time. Given that aims and justifications appear to have changed, but typical sample sizes have not, it is possible that some researchers choose sample size for pilot cRCTs primarily out of convenience or practical reasons, whilst stating other justifications.

KW - Cluster trials

KW - Pilot studies

KW - Sample sizes

UR - https://link.springer.com/article/10.1186/s40814-026-01792-z

U2 - 10.1186/s40814-026-01792-z

DO - 10.1186/s40814-026-01792-z

M3 - Review article

C2 - 41840651

SN - 2055-5784

VL - 12

JO - Pilot and Feasibility Studies

JF - Pilot and Feasibility Studies

IS - 1

M1 - 54

ER -

Sample size calculations for pilot cluster-randomised controlled trial: a review 2010-2020

Abstract

Bibliographical note

Keywords

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