The Cochrane Collaboration's tool for assessing risk of bias in randomised trials

Randomised trials, and systematic reviews of such trials, provide the most reliable evidence about the effects of healthcare interventions.Provided that there are enough participants, randomisation should ensure that participants in the intervention and comparison groups are similar with respect to both known and unknown prognostic factors.Differences in outcomes of interest between the different groups can then in principle be ascribed to the causal effect of the intervention. 1usal inferences from randomised trials can, however, be undermined by flaws in design, conduct, analyses, and reporting, leading to underestimation or overestimation of the true intervention effect (bias). 2 However, it is usually impossible to know the extent to which biases have affected the results of a particular trial.Systematic reviews aim to collate and synthesise all studies that meet prespecified eligibility criteria 3 using methods that attempt to minimise bias.To obtain reliable conclusions, review authors must carefully consider the potential limitations of the included studies.The notion of study "quality" is not well defined but relates to the extent to which its design, conduct, analysis, and presentation were appropriate to answer its research question.5][6][7] Until recently, Cochrane reviews used a variety of these tools, mainly checklists. 8In 2005 the Cochrane Collaboration's methods groups embarked on a new strategy for assessing the quality of randomised trials.In this paper we describe the collaboration's new risk of bias assessment tool, and the process by which it was developed and evaluated. Development of risk assessment toolIn May 2005, 16 statisticians, epidemiologists, and review authors attended a three day meeting to develop the new tool.Before the meeting, JPTH and DGA compiled an extensive list of potential sources of bias in clinical trials.The items on the list were divided into seven areas: generation of the allocation sequence; concealment of the allocation sequence; blinding; attrition and exclusions; other generic sources of bias; biases specific to the trial design (such as crossover or cluster randomised trials); and biases that might be specific to a clinical specialty.For each of the seven areas, a nominated meeting participant prepared a review of the empirical evidence, a discussion of specific issues and uncertainties, and a proposed set of criteria for assessing protection from bias as adequate, inadequate, or unclear, supported by examples.