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Updated: May 6, 2026

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

Rebecca M Simpson1, Jen Lewis2, Michael J Campbell2

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Pilot and Feasibility Studies
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PubMed
Summary
This summary is machine-generated.

Sample sizes for pilot cluster-randomised controlled trials (cRCTs) have remained constant from 2010-2020, despite evolving justifications. This suggests sample size choices may be driven by convenience rather than stated aims.

Keywords:
Cluster trialsPilot studiesSample sizes

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Area of Science:

  • Clinical Trials Methodology
  • Biostatistics
  • Health Services Research

Background:

  • No standardized guidance exists for pilot cluster-randomised controlled trials (cRCTs) sample sizes.
  • Pilot trials should focus on feasibility, not efficacy, and power calculations are inappropriate for sample size justification.
  • CONSORT guidelines require sample size justification for pilot trials.

Purpose of the Study:

  • To review sample size choices and justifications in pilot cRCTs.
  • To analyze trends in pilot cRCT sample sizes over time.
  • To explore the impact of CONSORT guideline publications on study practices.

Main Methods:

  • Systematic search of PubMed and Web of Science for pilot/feasibility cRCTs published from 2010 onwards.
  • Data extraction focused on planned sample size (clusters per arm), participants per arm, and enrolled sample size.
  • Descriptive and graphical analyses were employed.

Main Results:

  • 170 pilot or feasibility cRCTs were identified.
  • The median sample size was consistently four clusters per arm.
  • Sample size remained stable regardless of justification, planned analyses, or study characteristics.

Conclusions:

  • Pilot cRCT sample sizes have been static between 2010-2020.
  • Stated study aims and justifications for sample size do not appear to influence the chosen sample size.
  • Sample sizes may be determined by convenience or practical considerations rather than research objectives.