Randomisation to protect against selection bias in healthcare trials

• Published in: Cochrane database of systematic reviews 2011-04, Vol.2011 (4), p.MR000012-MR000012
• Main Authors: Odgaard-Jensen, Jan, Vist, Gunn E, Timmer, Antje, Kunz, Regina, Akl, Elie A, Schünemann, Holger, Briel, Matthias, Nordmann, Alain J, Pregno, Silvia, Oxman, Andrew D
• Format: Article
• Language: English
• Subjects: Clinical Trials as Topic - methods; Clinical Trials as Topic - standards; Clinical Trials as Topic - statistics & numerical data; Controlled Clinical Trials as Topic - methods; Controlled Clinical Trials as Topic - standards; Controlled Clinical Trials as Topic - statistics & numerical data; EVALUATION METHODOLOGY; Methodology; Random Allocation; Randomized Controlled Trials as Topic - methods; Randomized Controlled Trials as Topic - standards; Randomized Controlled Trials as Topic - statistics & numerical data; Selection Bias; Treatment Outcome
• ISSN: 1469-493X
• DOI: 10.1002/14651858.MR000012.pub3

Randomised trials use the play of chance to assign participants to comparison groups. The unpredictability of the process, if not subverted, should prevent systematic differences between comparison groups (selection bias). Differences due to chance will still occur and these are minimised by randomising a sufficiently large number of people. To assess the effects of randomisation and concealment of allocation on the results of healthcare studies. We searched the Cochrane Methodology Register, MEDLINE, SciSearch and reference lists up to September 2009. In addition, we screened articles citing included studies (ISI Science Citation Index) and papers related to included studies (PubMed). Eligible study designs were cohorts of studies, systematic reviews or meta-analyses of healthcare interventions that compared random allocation versus non-random allocation or adequate versus inadequate/unclear concealment of allocation in randomised trials. Outcomes of interest were the magnitude and direction of estimates of effect and imbalances in prognostic factors. We retrieved and assessed studies that appeared to meet the inclusion criteria independently. At least two review authors independently appraised methodological quality and extracted information. We prepared tabular summaries of the results for each comparison and assessed the results across studies qualitatively to identify common trends or discrepancies. A total of 18 studies (systematic reviews or meta-analyses) met our inclusion criteria. Ten compared random allocation versus non-random allocation and nine compared adequate versus inadequate or unclear concealment of allocation within controlled trials. All studies were at high risk of bias.For the comparison of randomised versus non-randomised studies, four comparisons yielded inconclusive results (differed between outcomes or different modes of analysis); three comparisons showed similar results for random and non-random allocation; two comparisons had larger estimates of effect in non-randomised studies than in randomised trials; and two comparisons had larger estimates of effect in randomised than in non-randomised studies.Five studies found larger estimates of effect in trials with inadequate concealment of allocation than in trials with adequate concealment. The four other studies did not find statistically significant differences. The results of randomised and non-randomised studies sometimes differed. In some instances non-randomised studies yiel