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Education And Debate Statistics Notes

Blinding in clinical trials and other studies

BMJ 2000; 321 doi: (Published 19 August 2000) Cite this as: BMJ 2000;321:504
  1. Simon J Daya, manager, clinical biometrics,
  2. Douglas G Altmanb, professor of statistics in medicine
  1. a Leo Pharmaceuticals, Princes Risborough, Buckinghamshire HP27 9RR
  2. b ICRF Medical Statistics Group, Institute of Health Sciences, Oxford OX3 7LF
  1. Correspondence to: S J Day

    Human behaviour is influenced by what we know or believe. In research there is a particular risk of expectation influencing findings, most obviously when there is some subjectivity in assessment, leading to biased results. Blinding (sometimes called masking) is used to try to eliminate such bias.

    It is a tenet of randomised controlled trials that the treatment allocation for each patient is not revealed until the patient has irrevocably been entered into the trial, to avoid selection bias. This sort of blinding, better referred to as allocation concealment, will be discussed in a future statistics note. In controlled trials the term blinding, and in particular “double blind,” usually refers to keeping study participants, those involved with their management, and those collecting and analysing clinical data unaware of the assigned treatment, so that they should not be influenced by that knowledge.

    The relevance of blinding will vary according to circumstances. Blinding patients to the treatment they have received in a controlled trial is particularly important when the …

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