Methodology State-of-the-art workshop: Choosing the target difference (“effect size”) for a randomised controlled trial

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Randomised controlled trials (RCT) are considered the optimal study design to assess comparative clinical efficacy and effectiveness. A sample size calculation ensures the study has a reasonable chance to achieve its pre-specified objectives: A compromise is required between the possibility of being misled by chance (there is no true difference between treatments) and the risk of not identifying a difference even when one of the treatments is truly superior. The “target difference” is the size of the difference the RCT is designed to reliably investigate. It is also commonly referred to as the trial “effect size”. The critical question is whether a particular difference detected statistically, is of any practical interest. From a scientific and ethical standpoint, selecting an appropriate targeted difference is of crucial importance - specifying too small a target difference could be a wasteful (and perhaps unethical) use of resources, and too large could lead to an important difference being overlooked. Furthermore, the resultant undersized study will not produce reliable evidence.Despite there being many different approaches available to choose the target difference, few are used regularly in practice. The project will: 1. Review the existing guidance; 2. Identify key methodological developments or changes in practice; 3. Determine the scope of the guidance required; 4. Produce structured guidance for choosing and reporting the target difference (effect size)