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

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.