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. However, until now, there has not been comprehensive guidance on how to specify this effect. 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.

Sample size calculations are central to the design of health research trials.