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Blinding: an essential component in decreasing risk of bias in experimental designs
  1. Dorothy Forbes
  1. Correspondence to: Dr Dorothy Forbes
    Faculty of Nursing, University of Alberta, Level 3, Edmonton Clinic Health Academy (ECHA), 11405-87Ave, Edmonton, Alberta, Canada T6G 1C9; dorothy.forbes{at}

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What is blinding?

Blinding (or masking) is the process used in experimental research by which study participants, persons caring for the participants, persons providing the intervention, data collectors and data analysts are kept unaware of group assignment (control vs intervention). Blinding aims to reduce the risk of bias that can be caused by an awareness of group assignment. With blinding, outcomes can be attributed to the intervention itself and not influenced by behaviour or assessment of outcomes that can result purely from knowledge of group allocation.

Why incorporate blinding?

Lack of blinding in randomised controlled trials (RCTs) has been shown to be associated with more exaggerated estimated intervention effects, by 9% on average.1 Studies with subjective outcomes are more likely to show these exaggerated estimates.2 In a systematic review of 250 RCTs, researchers observed a significant difference in the size of the estimated treatment effect between trials that reported ‘double-blinding’ compared with those that did not (p=0.01), with an overall OR 17% larger in studies that did not report blinding.3 In addition, studies are often simply described as ‘blind’ or ‘double-blind’ and do not specify who was blinded,4 which means blinding of one …

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