Risk of bias of randomized trials over time
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OBJECTIVES: To determine the variation in the risk of bias (RoB) of randomized controlled trials (RCTs) in time. STUDY DESIGN AND SETTING: We reviewed all included RCTs from systematic reviews (SRs) published in the issue 12 (2012) of the Cochrane Databases of Systematic Reviews. We extracted the RoB author's evaluation per domain and other RCT characteristics. Multivariate logistic regression was used to evaluate association between the presence of a low RoB according to RoB domains and other characteristics. RESULTS: We included 1,732 RCTs from 97 SRs. The rates of RCTs judged as having low and high RoB significantly increased over time, whereas the rates of unclear RoB decreased for several domains. Increased rates of low RoB were consistent when considering the type of intervention (drugs vs. others), sample size, and country income level. Multivariate logistic regression shows that RCTs published between 2006 and 2012, compared with those published before 1990, were more likely to be considered at low RoB for sequence generation (odds ratio [OR] = 3.96; 95% confidence interval [CI]: 2.29, 6.87), allocation concealment (OR = 3.56; 95% CI: 1.96, 6.46), incomplete outcome data (objective outcomes; OR = 1.89; 95% CI: 1.13, 3.15), and selective reporting (OR = 4.14; 95% CI: 2.35, 7.29) domains. CONCLUSION: RCTs have improved reporting during the last decades decreasing the uncertainty for the RoB assessment.
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