Blinded trials taken to the test: an analysis of randomized clinical trials that report tests for the success of blinding

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.