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Specifying the target difference in the primary outcome for a randomised controlled trial: guidance for researchers

Lookup NU author(s): Jenni Hislop, Professor Luke ValeORCiD

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


Abstract

Background: Central to the design of a randomised controlled trial is the calculation of the number of participants needed. This is typically achieved by specifying a target difference and calculating the corresponding sample size, which provides reassurance that the trial will have the required statistical power (at the planned statistical significance level) to identify whether a difference of a particular magnitude exists. Beyond pure statistical or scientific concerns, it is ethically imperative that an appropriate number of participants should be recruited. Despite the critical role of the target difference for the primary outcome in the design of randomised controlled trials, its determination has received surprisingly little attention. This article provides guidance on the specification of the target difference for the primary outcome in a sample size calculation for a two parallel group randomised controlled trial with a superiority question.Methods: This work was part of the DELTA (Difference ELicitation in TriAls) project. Draft guidance was developed by the project steering and advisory groups utilising the results of the systematic review and surveys. Findings were circulated and presented to members of the combined group at a face-to-face meeting, along with a proposed outline of the guidance document structure, containing recommendations and reporting items for a trial protocol and report. The guidance and was subsequently drafted and circulated for further comment before finalisation.Results: Guidance on specification of a target difference in the primary outcome for a two group parallel randomised controlled trial was produced. Additionally, a list of reporting items for protocols and trial reports was generated.Conclusions: Specification of the target difference for the primary outcome is a key component of a randomized controlled trial sample size calculation. There is a need for better justification of the target difference and reporting of its specification.


Publication metadata

Author(s): Cook JA, Hislop J, Altman DG, Fayers P, Briggs AH, Ramsay CR, Norrie JD, Harvey IM, Buckley B, Fergusson D, Ford I, Vale LD, Delta Group

Publication type: Article

Publication status: Published

Journal: Trials

Year: 2015

Volume: 16

Issue: 12

Print publication date: 01/01/2015

Online publication date: 15/01/2015

Acceptance date: 19/12/2014

Date deposited: 27/02/2015

ISSN (electronic): 1745-6215

Publisher: BioMed Central Ltd.

URL: http://dx.doi.org/10.1186/s13063-014-0526-8

DOI: 10.1186/s13063-014-0526-8


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Funding

Funder referenceFunder name
Chief Scientist Office of the Scottish Government Health and Social Care Directorates
06/98/01Health Technology Assessment (HTA)
G0902147MRC/NIHR Methodology Research Panel - MRC

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