Allocation concealment and blinding: when ignorance is bliss

Vance W Berger
Med J Aust 2005; 183 (3): . || doi: 10.5694/j.1326-5377.2005.tb06974.x
Published online: 1 August 2005

To the Editor: Forder et al conveyed that trials without allocation concealment have the potential to mislead.1 However, it is not true in any meaningful sense that “Without exception, allocation concealment is achievable in all randomised clinical trials. In contrast, it is not always possible to blind people to study treatments received.” Rather, “Masking may be defined as either the process (researchers not revealing treatment codes until the database is locked) or the result (complete ignorance of all trial participants as to which patients received which treatments). A masking claim indicates only the former . . . If masking is possible only some of the time, then clearly reference is being made to the result, and not the process. To be fair, then, one would have to ask if the result of allocation concealment is always possible . . . only the process of allocation concealment, but not its result, can be ensured.”2

  • National Cancer Institute, University of Maryland, Baltimore County, 6130 Executive Boulevard, MSC 7354, Bethesda, MD 20892, USA.


  • 1. Forder PM, Gebski VJ, Keech AC. Allocation concealment and blinding: when ignorance is bliss. Med J Aust 2005; 182: 87-89. <MJA full text>
  • 2. Berger VW, Christophi CA. Randomization technique, allocation concealment, masking, and susceptibility of trials to selection bias. J Mod Appl of Stat Methods 2003 2: 80-86.
  • 3. Berger VW, Ivanova A, Deloria-Knoll M. Minimizing predictability while retaining balance through the use of less restrictive randomization procedures. Stat Med 2003; 22: 3017-3028.
  • 4. Berger VW. Quantifying the magnitude of baseline covariate imbalances resulting from selection bias in randomized clinical trials (with discussion). Biometr J 2005; 47: 119-139.
  • 5. Berger VW. Selection bias and covariate imbalances in clinical trials. Chichester: John Wiley & Sons, 2005.


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