IJE Advance Access originally published online on April 17, 2007
International Journal of Epidemiology 2007 36(3):654-663; doi:10.1093/ije/dym020
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Blinded trials taken to the test: an analysis of randomized clinical trials that report tests for the success of blinding
1The Nordic Cochrane Centre, Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen Ø, Denmark
2Department of Orthopedic Surgery, Amager University Hospital, Italiensvej 1, DK-2300 Copenhagen S, Denmark
* Corresponding author: The Nordic Cochrane Centre, Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen Ø, Denmark. E-mail: ah{at}cochrane.dk
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Abstract |
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Background Blinding can reduce bias in randomized clinical trials, but blinding procedures may be unsuccessful. Our aim was to assess how often randomized clinical trials test the success of blinding, the methods involved and how often blinding is reported as being successful.
Methods We analysed a random sample of blinded randomized clinical trials indexed in the The Cochrane Central Register of Controlled Trials and published in 2001. We identified 1599 blinded trials, and noted if they had conducted any test for the success of blinding. We also selected 200 trials randomly that did not report any such test, and sent a questionnaire to the corresponding authors asking them if they had conducted any tests.
Results Thirty-one out of 1599 trials (2%) reported tests for the success of blinding. Test methods varied, and reporting was generally incomplete. Blinding was considered successful in 14 out of the 31 trials (45%) and unclear in 10 (32%). Of the seven trials (23%) reporting unsuccessful blinding the risk of a biased trial result was either not addressed or was discounted in six cases. We received 130 questionnaires from trial authors (65%) of which 15 (12%) informed that they had conducted, but not published, tests.
Conclusions Blinding is rarely tested. Test methods vary, and the reporting of tests, and test results, is incomplete. There is a considerable methodological uncertainty how best to assess blinding, and an urgent need for improved methodology and improved reporting.
Keywords Double-Blind Method, Statistical Data Interpretation, Randomized Controlled Trials/*methods/standards
Accepted 30 January 2007
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