A review of blinding in randomized controlled trials found results inconsistent and questionable

Background and Objective: To determine methods to assess the success of blinding in randomized controlled trials (RCTs). Methods: We searched MEDLINE, the Cochrane Controlled Trials Register, and the Cochrane Method Register and performed a manual search to target studies that attempt to assess blinding and describe the methods used in those studies. Results: A total of 90 reports were selected. Reports assessed the success of blinding participants (n = 58), care providers (n = 36), and outcome assessors (n = 15). Of the 58 reports assessing the success of blinding participants, 54 (93%) reported asking participants to guess their treatment assignment. There was no consistency in timing of assessment (e.g., once at the end of the trial, 57%, or several times during the trial, 26%) or modalities of answering (e.g., "do not know" answers, 43%, or participants forced to guess, 31%). A statistical analysis was performed in 57% of reports. The statistical analysis mainly compared the proportion of correct guesses to those produced by chance (32%) or checked for a relation between participants' guesses and treatment assignment (23%). Conclusions: Methods of assessing the success of blinding, analysis and reporting the results were inconsistent and questionable. (c) 2005 Elsevier Inc. All rights reserved.