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For definitive randomized controlled trials (RCTs), external pilot studies require at least 70 subjects for continuous outcomes and 60-100 for binary outcomes. Larger pilot studies are more efficient than inflating estimates for precision.

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Area of Science:

  • Clinical Trials Methodology
  • Biostatistics
  • Research Design

Background:

  • External pilot and feasibility studies inform definitive randomized controlled trial (RCT) design.
  • Lack of consensus exists on optimal pilot study size.
  • Inflating estimates due to imprecision is a common but potentially inefficient practice.

Purpose of the Study:

  • To illustrate the impact of pilot study sample size on the precision of parameter estimates.
  • To evaluate the consequences of inflating estimates for definitive RCT planning.
  • To provide evidence-based sample size recommendations for pilot studies.

Main Methods:

  • Simulation approach to model sampling distributions of standard deviation (continuous) and event rate (binary).
  • Assessment of precision, bias, and predicted power with increasing pilot sample sizes.
  • Evaluation of the 'confidence interval argument' for inflating estimates.

Main Results:

  • For continuous outcomes, precision gains diminish after 70 subjects (35 per group).
  • For binary outcomes (0.1-0.5 proportion), precision gains diminish after 60 subjects.
  • Inflating estimates can lead to excessively large definitive RCTs, requiring pilot sizes of 60-90.

Conclusions:

  • Recommend pilot studies with at least 70 subjects (35/group) for continuous outcomes (SDp estimation).
  • Recommend pilot studies with 60-100 subjects for binary outcomes (event rate estimation).
  • Larger pilot studies are more efficient than inflating estimates to account for imprecision.