Elsevier

The Lancet

Volume 359, Issue 9307, 23 February 2002, Pages 696-700
The Lancet

Series
Blinding in randomised trials: hiding who got what

https://doi.org/10.1016/S0140-6736(02)07816-9Get rights and content

Summary

Blinding embodies a rich history spanning over two centuries. Most researchers worldwide understand blinding terminology, but confusion lurks beyond a general comprehension. Terms such as single blind, double blind, and triple blind mean different things to different people. Moreover, many medical researchers confuse blinding with allocation concealment. Such confusion indicates misunderstandings of both. The term blinding refers to keeping trial participants, investigators (usually health-care providers), or assessors (those collecting outcome data) unaware of the assigned intervention, so that they will not be influenced by that knowledge. Blinding usually reduces differential assessment of outcomes (information bias), but can also improve compliance and retention of trial participants while reducing biased supplemental care or treatment (sometimes called co-intervention). Many investigators and readers naively consider a randomised trial as high quality simply because it is double blind, as if double-blinding is the sine qua non of a randomised controlled trial. Although double blinding (blinding investigators, participants, and outcome assessors) indicates a strong design, trials that are not double blinded should not automatically be deemed inferior. Rather than solely relying on terminology like double blinding, researchers should explicitly state who was blinded, and how. We recommend placing greater credence in results when investigators at least blind outcome assessments, except with objective outcomes, such as death, which leave little room for bias. If investigators properly report their blinding efforts, readers can judge them. Unfortunately, many articles do not contain proper reporting. If an article claims blinding without any accompanying clarification, readers should remain sceptical about its effect on bias reduction.

Section snippets

Potential effects of blinding

If participants are not blinded, knowledge of group assignment can affect responses to the intervention received.3 Participants who know that they have been assigned to a group who will receive a new treatment might harbour favourable expectations or increased apprehension. Those assigned to standard treatment, however, might feel deprived or relieved. Despite evidence to suggest that new treatments are as likely to be worse as they are to be better than standard treatments,6 participants

Lexicon of blinding

Non-blinded (open or open label) denotes trials in which everyone involved knows who has received which interventions throughout the trial. Blinding (masking) indicates that knowledge of the intervention assignments is hidden from participants, trial investigators, or assessors.

The terminology single blind usually means that one of the three categories of individuals (normally participant rather than investigator) remains unaware of intervention assignments throughout the trial.9 A single-blind

Masking or blinding

Some people prefer the term masking to blinding to describe the same procedure. Masking might be more appropriate in trials that involve participants who have impaired vision, and could be less confusing in trials in which blindness is an outcome.3 Blinding, however, conveys a strong bias prevention message. Apparently, blinding terminology emerged when Benjamin Franklin and colleagues19 actually blindfolded participants to shield them from knowledge in their assessments of the therapeutic

Placebos and blinding

Interventions (treatments) sometimes have no effect on the outcomes being studied.3 When an ineffective intervention is administered to participants in the context of a well-designed randomised controlled trial, however, beneficial effects on participants' attitudes sometimes occur, which in turn affect outcomes.10 Researchers refer to this phenomena as the placebo effect.

A placebo refers to a pharmacologically inactive agent that investigators administer to participants in the control group of

Does blinding prevent bias?

Some investigators, readers, and editors overstate the importance of blinding in prevention of bias. Indeed, some consider a randomised trial as high quality if it is double blind—ie, as if double blinding is the sine qua non of a randomised controlled trial.3 Unfortunately, scientific life is not that simple. A randomised trial can be methodologically sound and not be double blind or, conversely, double blind and not methodologically sound. Lasagna12 captured that notion long ago: “Let us

What to look for in descriptions of blinding

In general, if researchers describe a trial as double-blind, readers can assume that they have avoided bias. Empirical evidence lends support to this recommendation. As suggested in the CONSORT guidelines,24, 25 however, investigators should not use only the single-blind, double-blind, or triple-blind terminology, but should also explicitly state who was blinded, and how. Moreover, if the researchers contend that the trial investigators, participants, and assessors were blinded—ie, double

Conclusion

Blinding embodies a rich history spanning over two centuries. Most researchers worldwide understand blinding terminology, but confusion lurks beyond a general comprehension. Investigators should clearly explicate those blinded and not blinded in their trial, rather than only labeling their trial as single-blind, double-blind, or triple-blind. Readers should expect such clarity when reading and judging a trial report.

We thank Willard Cates and David L Sackett for their helpful comments on an

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