BMJ  2004;328:1135 (8 May), doi:10.1136/bmj.328.7448.1135-a

Letter

Turning a blind eye

Testing the success of blinding and the CONSORT statement

EDITOR—Reports of randomised trials should state clearly whether blinding was attempted, and if so who was blinded and how this was done.¹ Fergusson et al note that blinding may be ineffective in some trials, making them less sound methodologically than they seem to be.²

But trial participants asked to guess the treatment they received might well be influenced by outcome. We might expect to see an apparent breaking of the blind more often in trials when the effect of treatment was a marked, for either an intended outcome or adverse effect. Indeed, end of trial tests of blindness might be tests of hunches for adverse effects or efficacy.^{3 4} Assessments of blinding success would be much more reliable in trials when they can be carried out before the clinical outcome has been determined.

Furthermore, those who successfully decipher assignments may disguise their unblinding actions.^{3 4} That difficulty, along with the aforementioned interpretational difficulties, lead us to question the usefulness of blinding tests in some circumstances.

The CONSORT statement recommends reporting the findings of an assessment of blinding if it was done.⁵ Fergusson et al say 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. Furthermore, 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.

Cancer Research UK Medical Statistics Group, Centre for Statistics in Medicine, Oxford OX3 7LF doug.altman{at}cancer.org.uk

Quantitative Sciences, Family Health International, Research Triangle Park, NC, USA

Chalmers Research Group, Children's Hospital of Eastern Ontario, University of Ottawa, Ottawa, ON, Canada

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Competing interests: None declared.

References

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.[CrossRef][ISI][Medline]
2. Fergusson D, Cranley Glass K, 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. (21 February.)[Abstract/Free Full Text]
3. Schulz KF, Chalmers I, Altman DG. The landscape and lexicon of blinding in randomized trials. Ann Intern Med 2002;136: 254-9.[Free Full Text]
4. Schulz KF, Grimes DA. Blinding in randomised trials: hiding who got what. Lancet 2002;359: 696-700.[CrossRef][ISI][Medline]
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.[Abstract/Free Full Text]

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

Dean Fergusson, Kathleen Cranley Glass, Duff Waring, and Stan Shapiro
BMJ 2004 328: 432. [Abstract] [Full Text] [PDF]

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