An article collection published in Trials.
A key step in the design of a randomized controlled trial (RCT) is the estimation of the number of participants needed. Despite the critical role of the target difference in the primary outcome in a conventional RCT sample size, the manner in which it is determined, has received relatively little attention. Additionally, guidance on the reporting of the sample size calculation for a RCT is sorely needed. This article collection addresses these gaps in the literature. It includes a summary of the new DELTA2 guidance for researchers and funder representatives on the specification and reporting of the target difference and the sample size calculation for an RCT. Further articles address how the DELTA2 guidance was developed, previous guidance on this topic, along with a review of practice in a cohort of RCTs and a commentary on the value of the new guidance.