Blinded experiment

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A blind or blinded-experiment is an experiment in which information is withheld (masked or blinded) from participants to reduce bias.[1] A blind can be imposed on any participant of an experiment, including subjects, researchers, technicians, data analysts, and evaluators. A participant who deduces information that has been masked to them becomes unblinded. Participants must be assessed for unblinding to determine how well a blind succeed. Metascientific research reveals that much confusion surrounds the topic of blinding. To describe an experiment's blinding, it is necessary to state who has been blinded to what information, and how well each blind succeeded; the commonly used terminology (single blind, double blind etc.) is ambiguous. Blinding is an important tool of the scientific method, and is used in many fields of research. In some fields, such as medicine, it is considered essential.[2] The opposite of a blinded trial is an open trial.


The first blind experiment was conducted by the French Academy of Sciences in 1784 to investigate the claims of mesmerism as proposed by Franz Mesmer. In the experiment, researcher blindfolded mesmerists and asked them to identify objects that the experimenters had previously filled with "vital fluid". The subjects were unable to do so. In 1817, The first blind experiment recorded to have occurred outside of a scientific setting compared the musical quality of a Stradivarius violin to one with a guitar-like design. A violinist played each instrument while a committee of scientists and musicians listened from another room so as to avoid prejudice.[3][4]

One early essay advocating the blinding of researchers came from Claude Bernard in the latter half of the 19th century.[vague] Bernard recommended that the observer of an experiment should not have knowledge of the hypothesis being tested. This suggestion contrasted starkly with the prevalent Enlightenment-era attitude that scientific observation can only be objectively valid when undertaken by a well-educated, informed scientist.[5] The first recorded double-blind study was conducted in 1907 by W. H. R. Rivers and H. N. Webber to investigate the effects of caffeine.[6] Double-blind methods came into prominence in the mid-20th century.[7]

Single-blind experiments[edit]

In a single-blind experiment, subjects are not informed of their treatment allocation. Other participants in the experiment may have access to this information.

Double-blind experiments[edit]

In a double-blind experiment, neither the participants nor the researchers know which participants belong to the control group and which belong to the test group. A record of which subjects belong to each group is kept by a third party, and is not revealed to the researchers until the study is over. A properly performed double-blind experiment eliminates both the placebo effect and observer bias. A double-blind experiment cannot be performed when the experimenters must know the full facts (for example, when comparing sham to real surgery). Double-blind experiments achieve a higher standard of scientific rigor than equivalent single-blind or non-blind experiments.

Triple-blind experiments[edit]

A triple-blind study is an extension of the double-blind design in which the committee monitoring response variables is also blind to the placement of the test subjects. A triple-blind study has the advantage of eliminating observer bias in the monitoring committee. This study design may not be possible in a trial where the monitoring committee has an ethical responsibility to ensure participant safety, as such a design may interfere with this prerogative.[8]

Common misunderstanding

Although commonly used in study reports, the terms "single-blinded", "double-blinded" or "triple-blinded" are sometimes understood and interpretated differently by different researchers [9]. For instance, "double-blinded" could mean the study statisticians and participants were blinded; or participants and outcome assessors were blinded; or participants and people offering the intervention were blinded, etc. Current reporting guidance recommends describing both who was blinded and how [10].


In medicine[edit]

Blinding is considered essential in medicine,[11] but is often difficult to achieve. In drug studies, side effects can alert some patients to the presence of the drug, which effectively unblinds the study. While this shortcoming can be overcome with an active placebo, such controls are rarely used in medicine. It is also difficult to compare surgical and non-surgical interventions in blind trials. In some cases sham surgery is necessary for the blinding process. A good clinical protocol ensures that blinding is as effective as possible within ethical and practical constrains.

In physics[edit]

Modern nuclear physics and particle physics experiments often involve large numbers of data analysts working together to extract quantitative data from complex datasets. In particular, the analysts want to report accurate systematic error estimates for all of their measurements; this is difficult or impossible if one of the errors is observer bias. To remove this bias, the experimenters devise blind analysis techniques, where the experimental result is hidden from the analysts until they've agreed—based on properties of the data set other than the final value—that the analysis techniques are fixed.

One example of a blind analysis occurs in neutrino experiments, like the Sudbury Neutrino Observatory, where the experimenters wish to report the total number N of neutrinos seen. The experimenters have preexisting expectations about what this number should be, and these expectations must not be allowed to bias the analysis. Therefore, the experimenters are allowed to see an unknown fraction f of the dataset. They use these data to understand the backgrounds, signal-detection efficiencies, detector resolutions, etc.. However, since no one knows the "blinding fraction" f, no one has preexisting expectations about the meaningless neutrino count N' = N × f in the visible data; therefore, the analysis does not introduce any bias into the final number N which is reported. Another blinding scheme is used in B meson analyses in experiments like BaBar and CDF; here, the crucial experimental parameter is a correlation between certain particle energies and decay times—which require an extremely complex and painstaking analysis—and particle charge signs, which are fairly trivial to measure. Analysts are allowed to work with all the energy and decay data, but are forbidden from seeing the sign of the charge, and thus are unable to see the correlation (if any). At the end of the experiment, the correct charge signs are revealed; the analysis software is run once (with no subjective human intervention), and the resulting numbers are published. Searches for rare events, like electron neutrinos in MiniBooNE or proton decay in Super-Kamiokande, require a different class of blinding schemes.

The "hidden" part of the experiment—the fraction f for SNO, the charge-sign database for CDF—is usually called the "blindness box". At the end of the analysis period, one is allowed to "unblind the data" and "open the box".

In psychology[edit]

Single-blind trials are especially prone to bias in psychology and social science research where the experimenter has an idea of what the outcome should be, as the experimenter may transfer bias onto the subject. In some cases, while blind experiments would be useful, they are impractical or unethical: although it would be informative to raise children under arbitrary experimental conditions, such as on a remote island with a fabricated enculturation, it is a violation of ethics and human rights.

In forensics[edit]

In a police photo lineup, an officer shows a group of photos to a witness or crime victim and asks him or her to pick out the suspect. This is basically a single-blind test of the witness's memory, and may be subject to subtle or overt influence by the officer. There is a growing movement in law enforcement to move to a double-blind procedure in which the officer who shows the photos to the witness does not know which photo is of the suspect.[12][13]

In music[edit]

Auditions for symphony orchestras take place behind a curtain so that the judges cannot see the performer. Blinding the judges to the gender of the performers has been shown to increase the hiring of women.[14]


  1. ^ Oxford English Dictionary, 2nd ed.
  2. ^ "Oxford Centre for Evidence-based Medicine - Levels of Evidence (March 2009) - CEBM". 11 June 2009. Archived from the original on 26 October 2017. Retrieved 2 May 2018.
  3. ^ Fétis F (1868). Biographie Universelle des Musiciens et Bibliographie Générale de la Musique, Tome 1 (Second ed.). Paris: Firmin Didot Frères, Fils, et Cie. p. 249. Retrieved 2011-07-21.
  4. ^ Dubourg G (1852). The Violin: Some Account of That Leading Instrument and its Most Eminent Professors... (Fourth ed.). London: Robert Cocks and Co. pp. 356–357. Retrieved 2011-07-21.
  5. ^ Daston L (2005). "Scientific Error and the Ethos of Belief". Social Research. 72 (1): 18.
  6. ^ Rivers WH, Webber HN (August 1907). "The action of caffeine on the capacity for muscular work". The Journal of Physiology. 36 (1): 33–47. doi:10.1113/jphysiol.1907.sp001215. PMC 1533733. PMID 16992882.
  7. ^ Alder K (2006). Kramer LS, Maza SC (eds.). A Companion to Western Historical Thought. The History of Science, Or, an Oxymoronic Theory of Relativistic Objectivity. Blackwell Companions to History. Wiley-Blackwell. p. 307. ISBN 978-1-4051-4961-7. Retrieved 2012-02-11. Shortly after the start of the Cold War [...] double-blind reviews became the norm for conducting scientific medical research, as well as the means by which peers evaluated scholarship, both in science and in history.
  8. ^ Friedman LM, Furberg CD, DeMets DL (2010). Fundamentals of Clinical Trials. New York: Springer. pp. 119–132. ISBN 978-1-4419-1585-6.
  9. ^ 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.
  10. ^ Moher D, Hopewell S, Schulz KF, Montori V, Gøtzsche PC, Devereaux PJ, Elbourne D, Egger M, Altman DG (March 2010). "CONSORT 2010 explanation and elaboration: updated guidelines for reporting parallel group randomised trials". BMJ. 340 (mar23 1): c869. doi:10.1136/bmj.c869. PMC 2844943. PMID 20332511.
  11. ^ "Oxford Centre for Evidence-based Medicine - Levels of Evidence (March 2009) - CEBM". 11 June 2009. Archived from the original on 26 October 2017. Retrieved 2 May 2018.
  12. ^ Dittmann M (July–August 2004). "Accuracy and the accused: Psychologists work with law enforcement on research-based improvements to crime-suspect identification". Monitor on Psychology. American Psychological Association. 35 (7): 74.
  13. ^ Koerner BI (July–August 2002). "Under the Microscope". Legal Affairs. Retrieved 2 May 2018.
  14. ^ Miller CC (25 February 2016). "Is Blind Hiring the Best Hiring?". The New York Times. Retrieved 26 April 2019.