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Double-blind study

A double-blind study is an experiment in which two parties are blinded to some information, most often both patients and researchers being blinded to treatment allocations[1]. Blinding means that information that could influence participants or investigators is withheld until the experiment is completed[2].

Blinding can be applied to different participants in an experiment, including study subjects, researchers, technicians, data analysts and outcome assessors[3]. It is used to reduce or eliminate potential sources of bias, such as participants' expectations, observer bias and confirmation bias[2]. However, the meaning of the terms single-blind, double-blind and triple-blind can vary across studies[1].

CONSORT guidelines state that these terms should no longer be used because they are ambiguous[4]. For instance, "double-blind" may mean that the data analysts and patients were blinded, that the patients and outcome assessors were blinded, or that the patients and those administering the intervention were blinded[4]. To describe blinding, it is necessary to report who has been blinded to what information, and how well each blind succeeded[4].

A participant becomes unblinded if they deduce or otherwise obtain information that has been masked from them[5]. For example, a patient who experiences a side effect may correctly guess their treatment[5]. Unblinding that occurs before the conclusion of a study is a source of bias, whereas unblinding after the study has concluded is not, because data collection and analysis are both complete[6][7]. A code-break procedure dictates when a subject should be unblinded prematurely, and it should allow this only in cases of emergency[6]. In pharmacological trials, an active placebo can reduce premature unblinding by ensuring that side effects are present in both groups[8].

Why it matters to investors

Bias due to poor blinding tends to favour the experimental group, resulting in an inflated effect size and a risk of false positives[9]. The success of blinding is rarely reported or measured, so it is implicitly assumed that experiments described as "blind" are truly blind[5].

Related: Randomized controlled trial, Phase 3 trial

Sources

  1. Schulz KF, Chalmers I, Altman DG (February 2002). "The landscape and lexicon of blinding in randomized trials". Annals of Internal Medicine . 136 (3): 254– 9. doi : 10.7326/0003-4819-136-3-200202050-00022 . PMID 11827510 . S2CID 34932997 . doi.org (via Wikipedia: Blinded experiment)
  2. Kaptchuk TJ (1998). "The double-blind randomized clinical trial". Intentional ignorance: A history of blind assessment and placebo controls in medicine . Vol. 72. Bulletin of the History of Medicine. pp. 389– 433. doi : 10.1353/bhm.1998.0159 . PMID 9744753 . doi.org (via Wikipedia: Blinded experiment)
  3. Schulz KF, Grimes DA (2002). "Blinding in randomised trials: hiding who got what". The Lancet . 359 (9307): 696– 700. doi : 10.1016/S0140-6736(02)07816-9 . PMID 11879884 . doi.org (via Wikipedia: Blinded experiment)
  4. Moher, David; Hopewell, Sally; Schulz, Kenneth F.; Montori, Victor; Gøtzsche, Peter C.; Devereaux, P. J.; Elbourne, Diana; Egger, Matthias; Altman, Douglas G. (23 March 2010). "CONSORT 2010 explanation and elaboration: updated guidelines for reporting parallel group randomised trials" . BMJ (Clinical Research Ed.) . 340 : c869. doi : 10.1136/bmj.c869 . ISSN 1756-1833 . PMC 2844943 . PMID 20332511 ncbi.nlm.nih.gov (via Wikipedia: Blinded experiment)
  5. Bello, Segun; Moustgaard, Helene; Hróbjartsson, Asbjørn (October 2014). "The risk of unblinding was infrequently and incompletely reported in 300 randomized clinical trial publications". Journal of Clinical Epidemiology . 67 (10): 1059– 1069. doi : 10.1016/j.jclinepi.2014.05.007 . ISSN 1878-5921 . PMID 24973822 . doi.org (via Wikipedia: Blinded experiment)
  6. Quittell, Lynne M. (3 October 2018). "The Scientific and Social Implications of Unblinding a Study Subject". The American Journal of Bioethics . 18 (10): 71– 73. doi : 10.1080/15265161.2018.1513589 . ISSN 1526-5161 . PMID 30339067 . S2CID 53014880 . doi.org (via Wikipedia: Blinded experiment)
  7. Dinnett EM, Mungall MM, Kent JA, Ronald ES, McIntyre KE, Anderson E, Gaw A (2005). "Unblinding of trial participants to their treatment allocation: lessons from the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER)". Clin Trials . 2 (3): 254– 259. doi : 10.1191/1740774505cn089oa . PMID 16279148 . S2CID 36252366 . doi.org (via Wikipedia: Blinded experiment)
  8. Double, D. B. (19 October 1996). "Placebo mania. Placebo-controlled trials are needed to provide data on the effectiveness of active treatment" . BMJ: British Medical Journal . 313 (7063): 1008– 9. doi : 10.1136/bmj.313.7063.1008b . ISSN 0959-8138 . PMC 2352320 . PMID 8892442 . ncbi.nlm.nih.gov (via Wikipedia: Blinded experiment)
  9. Moher, David; Altman, Douglas G.; Schulz, Kenneth F. (24 March 2010). "CONSORT 2010 Statement: updated guidelines for reporting parallel group randomised trials" . BMJ . 340 : c332. doi : 10.1136/bmj.c332 . ISSN 0959-8138 . PMC 2844940 . PMID 20332509 . ncbi.nlm.nih.gov (via Wikipedia: Blinded experiment)

Checked 8 October 2026. How we check sources