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Blindsided: challenging the dogma of masking in clinical trials

BMJ 2020; 368 doi: (Published 21 January 2020) Cite this as: BMJ 2020;368:m229

Linked Research

Impact of blinding on estimated treatment effects in randomised clinical trials

Linked Analysis

Fool’s gold? Why blinded trials are not always best

  1. Aaron M Drucker, assistant professor12,
  2. An-Wen Chan, associate professor12
  1. 1Women’s College Research Institute, Women’s College Hospital, 76 Grenville Street, Toronto, ON M5S1B2, Canada
  2. 2Division of Dermatology, Department of Medicine, University of Toronto, Toronto, ON, Canada
  1. Correspondence to A-W Chan anwen.chan{at}

Fresh evidence should encourage critical thinking about when blinding really matters

Evidence based medicine compels us to challenge assumptions and dogma. Health interventions should be subject to rigorous assessment—ideally in well conducted randomised trials. But what of the dogma of evidence based medicine itself? Blinding, also called masking, of participants, study staff, and outcome assessors is considered crucial to avoid performance and detection biases, and it is a key element of the Cochrane risk of bias tool for randomised trials.1 However, the linked comprehensive meta-epidemiological study by Moustgaard and colleagues (doi:10.1136/bmj.l6802) challenges the dogma that blinding is always necessary to protect against bias in trials.2

The MetaBLIND study compared the results of trials that assessed the same treatments and outcomes but differed with regard to blinding of patients, healthcare providers, or outcome assessors. Surprisingly, lack of blinding was not associated with an exaggeration of treatment effects compared with use of blinding. These findings were consistent across patient and investigator reported outcomes of varying degrees of subjectivity and were robust to sensitivity analyses …

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