Abstract
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.
| Original language | English |
|---|---|
| Number of pages | 118 |
| Journal | Health Technology Assessment |
| Volume | 23 |
| Issue number | 60 |
| DOIs | |
| Publication status | Published - 30 Oct 2019 |
Bibliographical note
Acknowledgements This project was funded by the MRC NIHR Methodology Research Programme in the UK in response to a commissioned call to lead a workshop on this topic in order to produce guidance. The members of the original DELTA2 group were Associate Professor Jonathan Cook, Professor Douglas Altman, Dr Jesse Berlin, Professor Martin Bland, Professor Richard Emsley, Dr Dean Fergusson, Dr Lisa Hampson, Professor Catherine Hewitt, Professor Craig Ramsay, Miss Joanne Rothwell, Dr Robert Smith, Dr William Sones, Professor Luke Vale, Professor Stephen Walters and Professor Steve Julious. As part of the process of developing this document, a 2-day workshop was held in Oxford in September 2016. The workshop participants were Professor Douglas Altman, Professor David Armstrong, Professor Deborah Ashby, Professor Martin Bland, Dr Andrew Cook, Professor Jonathan Cook, Dr David Crosby, Professor Richard Emsley, Dr Dean Fergusson, Professor Andrew Grieve, Dr Lisa Hampson, Professor Catherine Hewitt, Professor Steve Julious, Professor Graeme MacLennan, Professor Tim Maughan, Professor Jon Nicholl, Dr José Pinheiro, Professor Craig Ramsay, Miss Joanne Rothwell, Dr William Sones, Professor Nigel Stallard, Professor Luke Vale, Professor Stephen Walters and Dr Ed Wilson. The authors would like to acknowledge and thank the participants in the Delphi exercise and the one-off engagement sessions with various groups, including the SCT, PSI and JSM conference session attendees, along with the other workshop participants who kindly provided helpful input and comments on the scope and content of this document. We would also like to thank, in particular, Dr Robert Smith in his role as a member of the public who provided a helpful public perspective during the workshop and in the development and revision of this document. Finally, the authors would like to thank Stefano Vezzoli for in-depth comments that helped to refine this document, helpful feedback from the MRC Methodological Research Programme Advisory Group, and representatives of the Medicines and Healthcare products Regulatory Agency and Health and Social Care, Northern Ireland. Dedication This work is dedicated to Douglas Altman, an inspirational researcher, friend and colleague. Contributions of authors Jonathan A Cook (Professor) drafted the initial version of the manuscript, and read and approved the final version. Steven A Julious (Professor) drafted the initial version of the manuscript and read and approved the final version. William Sones (Statistician) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Lisa V Hampson (Associate Director) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Catherine Hewitt (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Jesse A Berlin (Vice President and Global Head of Epidemiology) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Deborah Ashby (Co-Director) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Richard Emsley (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Dean A Fergusson (Senior Scientist and Director) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Stephen J Walters (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Edward CF Wilson (Senior Research Associate in Health Economics) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Graeme MacLennan (Director and Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Nigel Stallard (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Joanne C Rothwell (PhD student) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Martin Bland (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Louise Brown (Senior Statistician) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Craig R Ramsay (Director and Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Andrew Cook (Consultant in Public Health Medicine and Fellow in Health Technology Assessment) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. David Armstrong (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Douglas Altman (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version. Luke D Vale (Professor) contributed to the development of the document, commented on the draft manuscript, and read and approved the final version.
Access to Document
- HTA_3030850_VOR
© Queen’s Printer and Controller of HMSO 2019. This work was produced by Cook et al. under the terms of a commissioning contract issued by the Secretary of State for Health and Social Care. This issue may be freely reproduced for the purposes of private research and study and extracts (or indeed, the full report) may be included in professional journals provided that suitable acknowledgement is made and the reproduction is not associated with any form of advertising. Applications for commercial reproduction should be addressed to: NIHR Journals Library, National Institute for Health Research, Evaluation, Trials and Studies Coordinating Centre, Alpha House, University of Southampton Science Park, Southampton SO16 7NS, UK.