A review of blinding in randomized controlled trials found results inconsistent and questionable.
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.
Boutron
Isabelle
I
INSERM, U738, Paris, France. isabelle.boutron@bch.ap-hop-paris.fr
Estellat
Candice
C
Ravaud
Philippe
P
D015706:Bibliometrics; D003627:Data Interpretation, Statistical; D004311:Double-Blind Method; D006801:Humans; D016032:Randomized Controlled Trials as Topic / Q000379:methods / Q000592:standards*; D012680:Sensitivity and Specificity