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Centre for Statistics in Medicine

DELTA2 Guidance: Project summary

The DELTA2 project aimed to develop guidance for specifying the target difference (or effect size) in the sample size calculation for a randomised clinical trial (RCT). This guidance is aimed at researchers and funders of RCTs. It is hoped that the guidance will help improve the design of future RCTs.



Project summary

Project members


Button stating: Funded by the NIHR

MRC logo


Experience of assessing RCT funding applications to the Medical Research Council (MRC) & National Institute for Health Research (NIHR) highlighted that the selection of the target difference (‘effect size’) was a key area of uncertainty in trial design.1 Proposals that used inappropriately large effect sizes were not uncommon. To address this problem, the MRC-NIHR Methodology Research Programme’s Advisory Group invited proposals for a Methodology State-of-the-Art Workshop to develop guidance on this aspect of trial design.


The DELTA2 project objectives were to:

  1. Review the existing guidance provided by funders to researchers and panel/board members;
  2. Identify any key methodological developments or changes in practice since a comprehensive review was undertaken and update method guidance;
  3. Determine the scope of the guidance that would aid researchers and address funders’ needs;
  4. Achieve consensus on structured guidance for choosing the target difference; and
  5. Identify future research needs.

To achieve these objectives, multiple stages of research were carried out. We reviewed the relevant guidance provided by UK and international funders of randomised trials. We also systematically reviewed recent methodological developments regarding methods for determining the target difference in randomised controlled trials.

A Delphi study involving key stakeholders was undertaken to assess their views on the required scope and focus of the guidance. The Delphi process incorporated a two-day workshop of key stakeholders to help us achieve consensus.


A draft version of the full guidance is available as a preprint. A summary of the guidance has been published and is available in the BMJ and Trials journals. Related papers are also available in an article collection curated by Trials. All outputs of the project are listed within these project pages here.


  1. Cook JA, Julious SA, Sones W, Rothwell JC, Ramsay CR, Hampson LV, Emsley R, Walters SJ, Hewitt C, Bland MJ, Fergusson DA, Berlin J, Altman D, Vale LD. Choosing the target difference ("effect size") for a randomised controlled trial - DELTA2 guidance protocol. Trials. 2017; 18: 271.