Who is blinded in randomized clinical trials? A study of 200 trials and a survey of authors

Background Insufficient blinding of persons involved in randomized clinical trials is associated with bias. The appraisal of the risk of bias is difficult without adequate information in trial reports.Purpose We wanted to study how blinding is reported in clinical trials and how lack of reporting relate to lack of blinding.Methods A cohort study of 200 blinded randomized clinical trials published in 2001 randomly sampled from the Cochrane Central Register of Controlled Trials, and a questionnaire survey of the trial authors.Results One-hundred and fifty-six (78%) articles described trials as ‘double blind’. In three (2%) of such articles the blinding status of patients, health care providers and data collectors was explicitly described. Eighty-eight (56%) articles did not describe the blinding status of any trial person, and 41 articles (26%) reported no blinding relevant information at all beyond the trial being ‘double blind’. One-hundred and thirty (65%) surveyed authors responded. Patients were blinde...