Testing for blinding is the only way to determine whether a trial is blind
To the Editor – The CONSORT 2010 Statement for reporting parallel
group randomised trials(1) and its accompanying ‘Explanation and
elaboration’(2) (currently online ahead of print) have recently been
published in BMJ and elsewhere. Notably, the item concerning blinding
(Item 11) no longer recommends that trialists report on whether or not
blinding has been maintained. We believe that this significant change is
unwarranted and imprudent, as it ignores a growing body of research that
stresses the importance of evaluating blinding(3-9).
The value of blinding is uncontroversial and is noted in the
explanation and elaboration of the CONSORT 2010 Statement. The controversy
lies in how the success of blinding should be assessed. In contrast to the
previous revision(10), the CONSORT 2010 Statement no longer advocates
reporting on the success of blinding via assessing guesses about
participants’ treatment allocation and comparing these guesses across the
treatment arms. The rationale provided by the authors for this is that
“because participants and healthcare providers will usually know whether
the participant has experienced the primary outcome, this makes it
difficult to determine if their responses reflect failure of blinding or
accurate assumptions about the efficacy of the intervention”(2: p13).
These two alternatives are not, however, mutually exclusive. Accurate
assumptions about the efficacy of the intervention are a possible reason
that blinding may fail, not an alternative to failed blinding.
For example, a participant may assume that Treatment X is superior to
Treatment Y and know that she has been allocated to receive one of those
two treatments. If that participant experiences and notices improvement as
a result of her treatment, then she will likely believe that she has been
allocated to Treatment X. If another participant holding the same
assumption experiences no improvement as a result of her treatment, then
he will likely believe that he has been allocated to Treatment Y. If this
pattern is repeated for other participants in the trial and Treatment X is
actually superior to treatment Y, then a test of blinding will almost
certainly reveal that blinding has failed. The CONSORT 2010 Statement
authors(1-2) suggest that this invalidates the test of blinding. However,
a test of blinding simply evaluates the pattern of guesses about treatment
allocation across treatment groups. While the reason that blinding has
failed in this case is due to the improvement that some but not other
participants experienced, it does not alter the fact that the pattern of
participants’ guesses about their treatment allocation differed across
treatment groups and that these differences may introduce bias into the
trial. The same applies for healthcare providers, assessors, and others
involved in the trial.
The CONSORT 2010 Statement(1) proposes that trialists should explain
the measures adopted to attempt to blind participants, healthcare
providers, and assessors more fully as an alternative to testing for
blinding. Explaining the method adopted to attempt to blind participants
no better indicates whether blinding was successful or not than explaining
the design of a RCT indicates whether one treatment is more effective than
the other. To achieve both, trialists must assess the outcomes of these
procedures. As far as blinding is concerned, comparing guesses about
treatment allocation is the only valid way to achieve this.
The fact that guesses about treatment allocation are influenced by
whether or not participants observe improvement does, of course, lead to
an unsatisfactory situation whereby more efficacious treatments will often
lead to failed blinding(4,8,11). This is not, however, a failure of tests
of blinding, but rather a methodological limitation to the process of
blinding. Avoiding reporting on tests of blinding simply ignores, rather
than addresses this limitation. Even worse, avoiding reporting on tests of
blinding provides trialists and readers with no information about whether
or not the results may have been biased by guesses about treatment
Testing for blinding is the only valid way to determine whether a
trial is blind. Trialsists conducing RCTs should, therefore, report on the
success of blinding. In situations where blinding is successful, trialists
and readers can be confident that guesses about treatment allocation have
not biased the trial’s outcome. In situations where blinding fails,
trialists and readers will have to evaluate whether or not bias may have
influenced the trial’s outcomes. Importantly, however, in the second
situation, while trialists are unable to label their trial as blind, the
failure of blinding should not be taken as definitive evidence that bias
occurred. Instead, trialists should provide a rationale as to why the test
of blinding was unsuccessful and a statement on whether or not they
consider the differences between treatment arms to be valid.
1. Schulz KF, Altman DG, Moher D, for the CONSORT Group. CONSORT 2010
Statement: updated guidelines for reporting parallel group randomised
trials. BMJ 2010;340:698-702.
2. Moher D, Hopewell S, Schulz KF, Montori V, Gotzsche PC, Devereaux
PJ, et al. CONSORT 2010 Explanation and Elaboration: updated guidelines
for reporting parallel group randomised trials. BMJ 2010;340(mar23_1):c869
3. Benedetti F. The importance of considering the effects of
perceived group assignment in placebo-controlled trials. Eval. Health
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effects in double-blind RCTs: An experimental analysis. Psychopharmacology
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their impact on clinical trials and clinical practice. Pain 2005;114(1-
6. Park J, Bang H, Canette I, Park J, Bang H, Canette I. Blinding in
clinical trials, time to do it better. Complement. Ther. Med.
7. Karanicolas PJ, Bhandari M, Taromi B, Akl EA, Bassler D, Alonso-
Coello P, et al. Blinding of outcomes in trials of orthopaedic trauma: an
opportunity to enhance the validity of clinical trials. Journal of Bone
& Joint Surgery - American Volume 2008;90(5):1026-33.
8. Colagiuri B, Morley KC, Boakes RA, Haber PS. Expectancy in double-
blind placebo-controlled trials: An example from alcohol dependence.
Psychother. Psychosom. 2009;78:167-71.
9. Hróbjartsson A, Forfang E, Haahr MT, Als-Nielsen B, Brorson S.
Blinded trials taken to the test: an analysis of randomized clinical
trials that report tests for the success of blinding. Int. J. Epidemiol.
10. Moher D, Schulz KF, Altman DG. The CONSORT Statement: Revised
recommendations for improving the quality of parallel-group randomized
trials. JAMA 2001;285(15):1987-81.
11. Sharpe L, Ryan B, Allard S, Sensky T. Testing for the integrity
of blinding in clinical trials: How valid are forced choice paradigms?
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Competing interests: No competing interests