New guidance to improve sample size calculations for trials: Eliciting the target difference Suzie Cro

Background: Sample size calculations are central to the design of health research trials. To ensure that the trial provides good evidence to answer the trial's research question, the target effect size (difference in means or proportions, odds ratio, relative risk or hazard ratio between trial arms) must be specified under the conventional approach to determining the sample size. However, until now, there has not been comprehensive guidance on how to specify this effect. Main text: This is a commentary on a collection of papers from two important projects, DELTA (Difference ELicitation in TriAls) and DELTA ² that aim to provide evidence-based guidance on systematically determining the target effect size, or difference and the resultant sample sizes for trials. In addition to surveying methods that researchers are using in practice, the research team met with various experts (statisticians, methodologists, clinicians and funders); reviewed guidelines from funding agencies; and reviewed recent methodological literature. The DELTA ² guidance stresses specifying important and realistic differences, and undertaking sensitivity analyses in calculating sample sizes. It gives recommendations on how to find appropriate differences, conduct the sample size calculation(s) and how to report these in grant applications, protocols and manuscripts. It is hoped that this will contribute not only to better powered studies, but better reporting and reproducibility and thinking about what a trial should be designed to achieve. Conclusions: The DELTA researchers have developed a set of comprehensive guidance documents that are welcome and will almost certainly improve the way that trials are designed and reported.