Blinding in randomized clinical trials: imposed impartiality

Clin Pharmacol Ther. 2011 Nov;90(5):732-6. doi: 10.1038/clpt.2011.207. Epub 2011 Oct 12.


Blinding, or "masking," is a crucial method for reducing bias in randomized clinical trials. In this paper, we review important methodological aspects of blinding, emphasizing terminology, reporting, bias mechanisms, empirical evidence, and the risk of unblinding. Theoretical considerations and empirical analyses support the blinding of patients, health-care providers, and outcome assessors as to the trial intervention to which patients have been allocated. We encourage extensive pretrial testing of blinding procedures and explicit reporting of who was in the blinded condition and the methods used to ensure blinding.

MeSH terms

  • Bias
  • Double-Blind Method*
  • Humans
  • Outcome Assessment, Health Care / methods*
  • Randomized Controlled Trials as Topic / methods*
  • Single-Blind Method*
  • Terminology as Topic