Background:

The randomised controlled trial is widely considered to be the gold standard study for comparing the effectiveness of health interventions. Central to its design is a calculation of the number of participants needed (the sample size) for the trial. The sample size is typically calculated by specifying the magnitude of the difference in the primary outcome between the intervention effects for the population of interest. This difference is called the ‘target difference’ and should be appropriate for the principal estimand of interest and determined by the primary aim of the study. The target difference between treatments should be considered realistic and/or important by one or more key stakeholder groups.

Objective:

The objective of the report is to provide practical help on the choice of target difference used in the sample size calculation for a randomised controlled trial for researchers and funder representatives.

Methods:

The Difference ELicitation in TriAls² (DELTA²) recommendations and advice were developed through a five-stage process, which included two literature reviews of existing funder guidance and recent methodological literature; a Delphi process to engage with a wider group of stakeholders; a 2-day workshop; and finalising the core document.

Results:

Advice is provided for definitive trials (Phase III/IV studies). Methods for choosing the target difference are reviewed. To aid those new to the topic, and to encourage better practice, 10 recommendations are made regarding choosing the target difference and undertaking a sample size calculation. Recommended reporting items for trial proposal, protocols and results papers under the conventional approach are also provided. Case studies reflecting different trial designs and covering different conditions are provided. Alternative trial designs and methods for choosing the sample size are also briefly considered.

Conclusions:

Choosing an appropriate sample size is crucial if a study is to inform clinical practice. The number of patients recruited into the trial needs to be sufficient to answer the objectives; however, the number should not be higher than necessary to avoid unnecessary burden on patients and wasting precious resources. The choice of the target difference is a key part of this process under the conventional approach to sample size calculations. This document provides advice and recommendations to improve practice and reporting regarding this aspect of trial design. Future work could extend the work to address other less common approaches to the sample size calculations, particularly in terms of appropriate reporting items.

Funding:

Funded by the Medical Research Council (MRC) UK and the National Institute for Health Research as part of the MRC–National Institute for Health Research Methodology Research programme.

Article history

This issue of the Health Technology Assessment journal series contains a project commissioned by the MRC–NIHR Methodology Research Programme (MRP). MRP aims to improve efficiency, quality and impact across the entire spectrum of biomedical and health-related research. In addition to the MRC and NIHR funding partners, MRP takes into account the needs of other stakeholders including the devolved administrations, industry R&D, and regulatory/advisory agencies and other public bodies. MRP supports investigator-led methodology research from across the UK that maximises benefits for researchers, patients and the general population – improving the methods available to ensure health research, decisions and policy are built on the best possible evidence.

To improve availability and uptake of methodological innovation, MRC and NIHR jointly supported a series of workshops to develop guidance in specified areas of methodological controversy or uncertainty (Methodology State-of-the-Art Workshop Programme). Workshops were commissioned by open calls for applications led by UK-based researchers. Workshop outputs are incorporated into this report, and MRC and NIHR endorse the methodological recommendations as state-of-the-art guidance at time of publication.

The authors have been wholly responsible for all data collection, analysis and interpretation, and for writing up their work. The HTA editors and publisher have tried to ensure the accuracy of the authors’ report and would like to thank the reviewers for their constructive comments on the draft document. However, they do not accept liability for damages or losses arising from material published in this report.

Declared competing interests of authors

Lisa V Hampson is an employee of Novartis Pharma AG (Basel, Switzerland) and reports grants from the Medical Research Council (MRC). Catherine Hewitt is a member of the National Institute for Health Research (NIHR) Health Technology Assessment (HTA) Commissioning Board since 2015. Jesse A Berlin is an employee of Johnson & Johnson (New Brunswick, NJ, USA) and holds shares in this company. Richard Emsley is a member of the NIHR HTA Clinical Trials Board since 2018. Deborah Ashby is a member of the HTA Commissioning Board, HTA Funding Boards Policy Group, HTA Mental Psychological and Occupational Health Methods Group, HTA Prioritisation Group and the HTA Remit and Competitiveness Group from January 2016 to December 2018. Stephen J Walters declares his department has contracts and/or research grants with the Department of Health and Social Care, NIHR, MRC and the National Institute for Health and Care Excellence. He also declares book royalties from John Wiley & Sons, Inc. (Hoboken, NJ, USA), as well as a grant from the MRC and personal fees for external examining. Louise Brown is a member of the NIHR Efficacy and Mechanism Evaluation Board since 2014. Craig R Ramsay is a member of the NIHR HTA General Board since 2017. Andrew Cook is a member of the NIHR HTA Interventional Procedures Methods Group, HTA Intellectual Property Panel, HTA Prioritisation Group, Public Health Research (PHR) Research Funding Board, Public Health Research Prioritisation Group and the PHR Programme Advisory Board.