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This document provides advice to improve practice and reporting regarding the target difference and sample size calculation in randomised controlled trials.

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Jonathan A CookORCID logo, Steven A JuliousORCID logo, William Sones, Lisa V Hampson, Catherine Hewitt, Jesse A Berlin, Deborah Ashby, Richard Emsley, Dean A FergussonORCID logo, Stephen J Walters, Edward CF Wilson, Graeme MacLennan, Nigel Stallard, Joanne C RothwellORCID logo, Martin Bland, Louise Brown, Craig R Ramsay, Andrew Cook, David ArmstrongORCID logo, Douglas Altman & Luke D Vale.

Jonathan A Cook 1,*, Steven A Julious 2, William Sones 1, Lisa V Hampson 3, Catherine Hewitt 4, Jesse A Berlin 5, Deborah Ashby 6, Richard Emsley 7, Dean A Fergusson 8, Stephen J Walters 2, Edward CF Wilson 9,10, Graeme MacLennan 11, Nigel Stallard 12, Joanne C Rothwell 2, Martin Bland 13, Louise Brown 14, Craig R Ramsay 15, Andrew Cook 16, David Armstrong 17, Douglas Altman 1,, Luke D Vale 18

1 Centre for Statistics in Medicine, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford, Oxford, UK
2 Medical Statistics Group, School of Health and Related Research, University of Sheffield, Sheffield, UK
3 Statistical Methodology and Consulting, Novartis Pharma AG, Basel, Switzerland
4 York Trials Unit, Department of Health Sciences, University of York, York, UK
5 Johnson & Johnson, Titusville, NJ, USA
6 Imperial Clinical Trials Unit, Imperial College London, London, UK
7 Department of Biostatistics and Health Informatics, Institute of Psychiatry, Psychology and Neuroscience, King’s College London, London, UK
8 Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, ON, Canada
9 Cambridge Centre for Health Services Research, Cambridge Clinical Trials Unit University of Cambridge, Cambridge, UK
10 Health Economics Group, Norwich Medical School, University of East Anglia, Norwich, UK
11 Centre for Healthcare Randomised Trials, University of Aberdeen, Aberdeen, UK
12 Warwick Medical School, Statistics and Epidemiology, University of Warwick, Coventry, UK
13 Department of Health Sciences, University of York, York, UK
14 MRC Clinical Trials Unit, Institute of Clinical Trials and Methodology, University College London, London, UK
15 Health Services Research Unit, University of Aberdeen, Aberdeen, UK
16 Wessex Institute, University of Southampton, Southampton, UK
17 School of Population Health and Environmental Sciences, King’s College London, London, UK
18 Health Economics Group, Institute of Health & Society, Newcastle University, Newcastle upon Tyne, UK
* Corresponding author Email:

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