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BMC Med Res Methodol. 2019 Aug 5;19(1):170. doi: 10.1186/s12874-019-0804-y.

Risk of bias judgments for random sequence generation in Cochrane systematic reviews were frequently not in line with Cochrane Handbook.

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Department of Surgery, University Hospital Split, Spinciceva 1, Split, Croatia.
Department for Research in Biomedicine and Health, University of Split, School of Medicine, Soltanska 2, Split, Croatia.
Department of Radiology, University Hospital Split, Spinciceva 1, Split, Croatia.
Department of Anesthesiology and Intensive Care, University Hospital Split, Spinciceva 1, Split, Croatia.
Center for Evidence-Based Medicine and Health Care, Catholic University of Croatia, Ilica 242, 10000, Zagreb, Croatia.



Assessing the risk of bias (RoB) in included studies is one of the key methodological aspects of systematic reviews. Cochrane systematic reviews appraise RoB of randomised controlled trials (RCTs) with the Cochrane RoB tool. Detailed instructions for using the Cochrane RoB tool are provided in the Cochrane Handbook for Systematic Reviews of Interventions (The Cochrane Handbook). The purpose of this study was to analyse whether Cochrane authors use adequate judgments about the RoB for random sequence generation of RCTs included in Cochrane reviews.


We extracted authors' judgments (high, low or unclear RoB) and supports for judgments (comments accompanying judgments which explain the rationale for a judgment) for random sequence generation of included RCTs from RoB tables of Cochrane reviews using automated data scraping. We categorised all supporting comments, analysed the number and type of various supporting comments and assessed adequacy of RoB judgment for randomisation in line with recommendations from the Cochrane Handbook.


We analysed 10,103 RCTs that were included in 704 Cochrane reviews. For 5,706 RCTs, randomisation was not described, but for the remaining RCTs, it was indicated that randomisation was performed using computer/software/internet (N = 2,850), random number table (N = 883), mechanical method (N = 359) or it was incomplete/inappropriate (N = 305). Overall, 1,220/10,103 trials (12%) did not have a RoB judgment in line with Cochrane Handbook guidance about randomisation. The highest proportion of misjudgements was found for trials with high RoB (28%), followed by those with low (20%) or unclear (3%). Therefore, one in eight judgments for the analysed domain in Cochrane reviews was not in line with Cochrane Handbook, and one in four if the judgment was "high risk".


Authors of Cochrane reviews often make judgments about the RoB related to random sequence generation that are not in line with instructions given in the Cochrane Handbook, which compromises the reliability of the systematic reviews. Our results can help authors of both Cochrane and non-Cochrane reviews which use Cochrane RoB tool to avoid making common mistakes when assessing RoB in included trials.


Cochrane; Randomisation; Risk of bias; Selection bias; Sequence generation; Systematic reviews

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