Explanation

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Blinding (masking) – the process of keeping the study group assignment hidden after allocation – is commonly used to reduce the risk of bias in randomised trials [356, 357]. Awareness of the intervention assigned to participants can introduce ascertainment bias in the measurement of outcomes, particularly subjective ones (e.g., quality of life) [326, 357, 358]; performance bias in the decision to discontinue or modify study interventions (e.g., dosing changes) (Item 15b), concomitant interventions, or other aspects of care (Item 15d) [359]; and exclusion/attrition bias in the decision to withdraw from the trial or to exclude a participant from the analysis [360, 361]. We use the generally preferred term “blinding”, but acknowledge that others prefer “masking” because “blind” also relates to an ophthalmologic condition and health outcome [362].

 

Many groups can be blinded: trial participants, care providers, data collectors, outcome assessors, data analysts [363], and manuscript writers. Blinding of data monitoring committee members is generally discouraged [364].

 

When blinding of trial participants and care providers is not possible due to obvious differences between the interventions [365, 366], blinding of the outcome assessors can often still be implemented [324]. The exception is in trials with patient/participant reported outcome (e.g. pain) where the patient is the outcome assessor.

 

It may also be possible to blind participants or trial personnel to the study hypothesis in terms of which intervention is considered active. For example, in a trial evaluating light therapy for depression, participants were informed that the study involved testing two different forms of light therapy, whereas the true hypothesis was that bright blue light was considered potentially effective and that dim red light was considered placebo [367].

 

Despite its importance, blinding has often been poorly described in trial protocols [5]. Yet, reviews of two samples of 108 and 292 trial protocols from 2016 found that 82% to 94% of protocols addressed the blinding status of participants and care providers, and 68% to 70% reported the blinding status of outcome assessors [9, 10].

 

The protocol should explicitly state who will be blinded to intervention groups – at a minimum, the blinding status of trial participants, care providers, and outcome assessors. Such a description is much preferred over the use of ambiguous terminology such as “single blind” or “double blind” [368, 369].

 

Summary of key elements to address

• Who will be blinded to treatment assignments:

  • Trial participants

  • Care providers (i.e., those administering the intervention)

  • Outcome assessors (i.e., those who determine whether a participant experienced the outcome of interest), e.g., the participant (for patient-reported outcomes), care provider, or  independent observer)

  • Data analysts performing the statistical analysis