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Logo of bmjThis ArticleThe BMJ
BMJ. 2007 May 5; 334(7600): 918.
PMCID: PMC1865441
Masking or Blinding?

Blinding is better than masking

Kenneth F Schulz, vice president of quantitative sciences,1 Douglas G Altman, director,2 and David Moher, director3

We agree with Morris et al that “blinding” terminology is probably inappropriate in ophthalmological settings.1 However, we disagree that these settings should ordain terminology for all randomised trials. They describe “masking” done in 1784 and provide dictionary definitions of masking and blinding to buttress their argument for using masking terminology. The techniques used in 1784, however, were not termed masking, and regular dictionaries do not adequately define methodological terms for clinical trials.

Blinding in clinical research enjoys a splendid history spanning over two centuries.2 Over the years it became entrenched in the tenets of medical research, and most researchers and readers grasp its meaning, although they have more difficulty understanding the different types of blinding.3 Evidently, “blinding” terminology surfaced when Antoine Lavoisier and Benjamin Franklin actually blindfolded (not masked) participants to shelter them from knowledge in their evaluations of the therapeutic claims made for mesmerism.4 The visual imagery of blindfolding, a complete covering of the eyes, conveys stronger bias avoidance than masking, where eye openings allow extensive viewing.5 Moreover, the International Conference on Harmonization (ICH) guidance primarily uses “blinding” terminology.4 The long history, pervasive general understanding, strong visual imagery, and adoption by the ICH lead us to suggest that “blinding” should remain the predominant terminology.6

However, we encourage authors to be more descriptive when describing the blinding used in the conduct of their randomised trial. For example, reporting that “participants and care providers were blinded” is more informative than simply stating “double blinding was used.” Moreover, with global electronic access to articles, if authors use “masking” they jeopardise communication. Medical researchers in Africa and Asia, for example, have little familiarity with masking terminology. Totally discarding blinding terminology seems imprudent.


Competing interests: None declared.


1. Morris D, Fraser S, Wormald R. Masking is better than blinding. BMJ 2007;334:799 (14 April.)
2. Kaptchuk TJ. Intentional ignorance: a history of blind assessment and placebo controls in medicine. Bull Hist Med 1998;72:389-433. [PubMed]
3. Devereaux PJ, Manns BJ, Ghali WA, Quan H, Lacchetti C, Montori VM, et al. Physician interpretations and textbook definitions of blinding terminology in randomized controlled trials. JAMA 2001;285(15):2000-3. [PubMed]
4. Franklin B, Bailly JS, Lavoisier A. Rapport des commissaires chargés par le roi, de l'examen du magnetisme animal. ed. A Nice: Chez Gabriel Floteron; 1785
5. Schulz KF, Grimes DA. Blinding in randomised trials: hiding who got what. Lancet 2002;359(9307):696-700. [PubMed]
6. Schulz KF, Chalmers I, Altman DG. The landscape and lexicon of blinding in randomized trials. Ann Intern Med 2002;136(3):254-9. [PubMed]

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