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Trials

, 16:405

First Online: 10 September 2015Received: 24 February 2015Accepted: 20 August 2015DOI: 10.1186-s13063-015-0920-x

Cite this article as: Kahan, B.C., Rehal, S. & Cro, S. Trials 2015 16: 405. doi:10.1186-s13063-015-0920-x

Abstract

BackgroundSelection bias occurs when recruiters selectively enrol patients into the trial based on what the next treatment allocation is likely to be. This can occur even if appropriate allocation concealment is used if recruiters can guess the next treatment assignment with some degree of accuracy. This typically occurs in unblinded trials when restricted randomisation is implemented to force the number of patients in each arm or within each centre to be the same. Several methods to reduce the risk of selection bias have been suggested; however, it is unclear how often these techniques are used in practice.

MethodsWe performed a review of published trials which were not blinded to assess whether they utilised methods for reducing the risk of selection bias. We assessed the following techniques: a blinding of recruiters; b use of simple randomisation; c avoidance of stratification by site when restricted randomisation is used; d avoidance of permuted blocks if stratification by site is used; and e incorporation of prognostic covariates into the randomisation procedure when restricted randomisation is used. We included parallel group, individually randomised phase III trials published in four general medical journals BMJ, Journal of the American Medical Association, The Lancet, and New England Journal of Medicine in 2010.

ResultsWe identified 152 eligible trials. Most trials 98 % provided no information on whether recruiters were blind to previous treatment allocations. Only 3 % of trials used simple randomisation; 63 % used some form of restricted randomisation, and 35 % did not state the method of randomisation. Overall, 44 % of trials were stratified by site of recruitment; 27 % were not, and 29 % did not report this information. Most trials that did stratify by site of recruitment used permuted blocks 58 %, and only 15 % reported using random block sizes. Many trials that used restricted randomisation also included prognostic covariates in the randomisation procedure 56 %.

ConclusionsThe risk of selection bias could not be ascertained for most trials due to poor reporting. Many trials which did provide details on the randomisation procedure were at risk of selection bias due to a poorly chosen randomisation methods. Techniques to reduce the risk of selection bias should be more widely implemented.

KeywordsRandomised controlled trial Clinical trial Selection bias Randomisation procedure AbbreviationsIQRinterquartile range

RCTrandomised controlled trial

SPIRITStandard Protocol Items: Recommendations for Interventional Trials

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Author: Brennan C. Kahan - Sunita Rehal - Suzie Cro

Source: https://link.springer.com/







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