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Practical help for specifying the target difference in sample size calculations for RCTs: the DELTA2 five-stage study, including a workshop

Lookup NU author(s): Professor Luke ValeORCiD


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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.This Difference ELicitation in TriAls2 (DELTA2) advice and recommendations document aims to help researchers choose the ‘target difference’ in a type of research study called a randomised controlled trial. The number of people needed to be involved in a study – the sample size – is usually based on a calculation aimed to ensure that the difference in benefit between treatments is likely to be detected. The calculation also accounts for the risk of a false-positive finding. No more patients than necessary should be involved. Choosing a ‘target difference’ is an important step in calculating the sample size. The target difference is defined as the amount of difference in the participants’ response to the treatments that we wish to detect. It is probably the most important piece of information used in the sample size calculation. How we decide what the target difference should be depends on various factors. One key decision to make is how we should measure the benefits that treatments offer. For example, if we are evaluating a treatment for high blood pressure, the obvious thing to focus on would be blood pressure. We could then proceed to consider what an important difference in blood pressure between treatments would be, based on experts’ views or evidence from previous research studies. This document seeks to provide assistance to researchers on how to choose the target difference when designing a trial. It also provides advice to help them clearly present what was done and why, when writing up the study proposal or reporting the study’s findings. The document is also intended to be read by those who decide whether or not a proposed study should be funded. Clarifying a study’s aim and getting a sensible sample size is important. It can affect not only those involved in the study, but also future patients who will receive treatment.

Publication metadata

Author(s): Cook JA, Julious SA, Sones W, Hampson LV, Hewitt C, Berlin JA, Ashby D, Emsley R, Fergusson DA, Walters SJ, Wilson EC, MacLennan G, Stallard N, Rothwell JC, Bland M, Brown L, Ramsay CR, Cook A, Armstrong D, Altman D, Vale LD

Publication type: Article

Publication status: Published

Journal: Health Technology Assessment (Winchester, England)

Year: 2019

Volume: 23

Issue: 60

Pages: 1-88

Online publication date: 01/10/2019

Acceptance date: 02/04/2019

ISSN (print): 1366-5278

ISSN (electronic): 2046-4924

Publisher: National Coordinating Centre for Health Technology Assessment


DOI: 10.3310/hta23600

PubMed id: 31661431


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