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Sources of bias in outcome assessment in randomised controlled trials : a case study.

Ainsworth, H. and Hewitt, C. and Torgerson, C. and Higgins, S. and Wiggins, A. and Torgerson, D. (2015) 'Sources of bias in outcome assessment in randomised controlled trials : a case study.', Educational research and evaluation., 21 (1). pp. 3-14.


Randomised controlled trials (RCTs) can be at risk of bias. Using data from a RCT we considered the impact of post-randomisation bias. We compared the trial primary outcome, which was administered blindly, with the secondary outcome which was not administered blindly. 522 children from 44 schools were randomised to receive a one-to-one maths tuition programme that was assessed using two outcome measures. The primary outcome measure was assessed blindly whilst the secondary outcome was delivered by the classroom teacher and therefore this was un-blinded. The effect sizes for primary and secondary outcomes were substantially different (0.33 and 1.11 respectively). Test questions that were similar between the two tests this did not explain the difference. There was greater heterogeneity between schools for the primary outcome, compared with the secondary outcome. We conclude that, in this trial, the difference between the primary and secondary outcomes was likely to have been due to lack of blinding of testers.

Item Type:Article
Keywords:Randomised trials, Methodology, Blinding, Treatment inherent measures.
Full text:(AM) Accepted Manuscript
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Publisher statement:This is an Accepted Manuscript of an article published by Taylor & Francis Group in Educational Research and Evaluation on 24/11/2014, available online at:
Date accepted:14 October 2014
Date deposited:06 November 2014
Date of first online publication:24 November 2014
Date first made open access:24 May 2016

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