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Research Methods & Reporting

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

BMJ 2011; 343 doi: (Published 18 October 2011) Cite this as: BMJ 2011;343:d5928
  1. Julian P T Higgins, senior statistician1,
  2. Douglas G Altman, director 2,
  3. Peter C Gøtzsche, director 3,
  4. Peter Jüni, head of division 4,
  5. David Moher, senior scientist56,
  6. Andrew D Oxman, senior researcher7,
  7. Jelena Savović, postdoctoral fellow8,
  8. Kenneth F Schulz, vice president9,
  9. Laura Weeks, research associate5,
  10. Jonathan A C Sterne, professor of medical statistics and epidemiology8
  11. Cochrane Bias Methods Group
  12. Cochrane Statistical Methods Group
  1. 1MRC Biostatistics Unit, Institute of Public Health, Cambridge CB2 0SR, UK
  2. 2Centre for Statistics in Medicine, University of Oxford, Oxford, UK
  3. 3The Nordic Cochrane Centre, Rigshospitalet and University of Copenhagen, Denmark
  4. 4Institute of Social and Preventive Medicine, University of Bern, Switzerland
  5. 5Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada
  6. 6Department of Epidemiology and Community Medicine, Faculty of Medicine, University of Ottawa, Canada
  7. 7Preventive and International Health Care Unit, Norwegian Knowledge Centre for the Health Services, Oslo, Norway
  8. 8Department of Social Medicine, University of Bristol, Bristol, UK
  9. 9FHI, Research Triangle Park, North Carolina, USA
  1. Correspondence to: J P T Higgins julian.higgins{at}
  • Accepted 22 July 2011

Flaws in the design, conduct, analysis, and reporting of randomised trials can cause the effect of an intervention to be underestimated or overestimated. The Cochrane Collaboration’s tool for assessing risk of bias aims to make the process clearer and more accurate

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.1

Causal 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 criteria3 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. Many tools for assessing the quality of randomised trials are available, including scales (which score the trials) and checklists (which assess trials without producing a score).4 5 6 7 Until recently, Cochrane reviews used a variety of these tools, mainly checklists.8 In 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 …

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