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Turning a blind eye: the success of blinding reported in a random sample of randomised, placebo controlled trials

BMJ 2004; 328 doi: https://doi.org/10.1136/bmj.328.74327.37952.631667.EE (Published 19 February 2004) Cite this as: BMJ 2004;328:432

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Testing the success of blinding and the CONSORT Statement

Reports of randomised trials should state clearly whether or not
blinding was attempted, and if so who was blinded and how this was
done.[1] Fergusson and colleagues present interesting findings regarding
the important question of whether blinding was effective.[2] As they say,
blinding may be ineffective in some trials, making them less sound
methodologically than they appear to be.

However, asking trial participants (or caregivers) to try to identify
the treatment received runs the risk that the ability to guess the
treatment received might well be influenced by outcome. For example, the
presence of adverse symptoms will surely influence participants asked
which treatment they think they were on, especially as the possibility of
such symptoms would surely have been mentioned as part of the informed
consent procedure.[3] In this case some apparent loss of blinding is
likely and should not be considered necessarily to be a weakness of the
trial. We might expect to see an apparent breaking of the blind more often
in trials where there was a marked treatment effect, for either an
intended outcome or adverse effect. Indeed, end-of-trial tests of
blindness might actually be tests of hunches for adverse effects or
efficacy.[4]. Assessments of blinding would clearly be much more reliable
in trials where they can be carried out before the clinical outcome has
been determined.

Furthermore, individuals may camouflage unblinding efforts. If
someone deciphers assignments, they may provide responses contrary to
their deciphering findings to disguise their unblinding actions.[4] That
difficulty and the aforementioned interpretational difficulties lead us
to question the usefulness of blinding tests in many circumstances.

The CONSORT Statement does not say that “the success of blinding is
to be reported in the publication.” Rather, it recommends reporting the
findings of an assessment of blinding if it was done[5]. Fergusson and
colleagues suggest that the CONSORT Statement should be amended to suggest
that assessment of blinding should be done routinely. We are not convinced
that all trialists should carry out such an exercise. Further, we do not
agree that the CONSORT Statement should be modified as suggested. CONSORT
is a set of reporting recommendations – it does not make statements on how
trials should be done, but asks that what was done should be fully and
accurately reported.

1 Moher D, Schulz KF, Altman DG for the CONSORT Group. The CONSORT
statement: revised recommendations for improving the quality of reports of
parallel-group randomised trials. Lancet
2001;357:1191–4.

2 Fergusson D, Glass KC, Waring D, Shapiro S. Turning a blind eye:
the success of blinding reported in a random sample of randomised, placebo
controlled trials. BMJ 2004; 328: 432-4.

3 Schulz KF, Chalmers I, Altman DG. The landscape and lexicon of
blinding in randomized trials. Ann Intern Med 2002;136:254-259.

4 Schulz KF, Grimes DA. Blinding in randomised trials: hiding who got
what. Lancet 2002;359:696-700.

5 Altman DG, Schulz KF, Moher D, Egger M, Davidoff F, Elbourne D, et
al. The revised CONSORT statement for reporting randomized trials:
explanation and elaboration. Ann Intern Med 2001;134:663-94.

Douglas G Altman
Cancer Research UK Medical Statistics Group, Centre for Statistics in
Medicine, Old Road Campus, Oxford

Kenneth F Schulz
Family Health International, Research Triangle Park, NC, USA

David Moher
University of Ottawa, Chalmers Research Group, Ottawa, Ontario, Canada

Competing interests:
The authors are the organisers of the CONSORT Group

Competing interests: No competing interests

21 February 2004
Douglas G. Altman
Professor of Statistics in Medicine
Kenneth F Schulz, David Moher
Cancer Research UK/NHS Centre for Statistics in Medicine, Oxford OX3 7LF