In ablind orblinded experiment, information which may influence the participants of theexperiment is withheld until after the experiment is complete. Good blinding can reduce or eliminate experimentalbiases that arise from a participants' expectations,observer's effect on the participants,observer bias,confirmation bias, and other sources. A blind can be imposed on any participant of an experiment, including subjects, researchers, technicians, data analysts, and evaluators. In some cases, while blinding would be useful, it is impossible or unethical. For example, it is not possible to blind a patient to their treatment in a physical therapy intervention. A goodclinical protocol ensures that blinding is as effective as possible within ethical and practical constraints.
During the course of an experiment, a participant becomesunblinded if they deduce or otherwise obtain information that has been masked to them. For example, a patient who experiences a side effect may correctly guess their treatment, becoming unblinded. Unblinding is common in blinded experiments, particularly in pharmacological trials. In particular, trials onpain medication andantidepressants are poorly blinded. Unblinding that occurs before the conclusion of a study is a source of experimental error, as the bias that was eliminated by blinding is re-introduced. TheCONSORT reporting guidelines recommend that all studies assess and report unblinding. In practice, very few studies do so.[1]
Blinding is an important tool of thescientific method, and is used in many fields of research. In some fields, such asmedicine, it is considered essential.[2] In clinical research, a trial that is not a blinded trial is called anopen trial.
The first known blind experiment was conducted by theFrench Royal Commission on Animal Magnetism in 1784 to investigate the claims ofmesmerism as proposed by Charles d'Eslon, a former associate ofFranz Mesmer. In the investigations, the researchers (physically) blindfolded mesmerists and asked them to identify objects that the experimenters had previously filled with "vital fluid". The subjects were unable to do so.[citation needed]
In 1817, the first blind experiment recorded to have occurred outside of a scientific setting compared the musical quality of aStradivarius 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]
An early example of a double-blind protocol was the Nuremberg salt test of 1835 performed by Friedrich Wilhelm von Hoven, Nuremberg's highest-ranking public health official,[5] as well as a close friend ofFriedrich Schiller.[6] This trial contested the effectiveness ofhomeopathic dilution.[5]
In 1865,Claude Bernard published hisIntroduction to the Study of Experimental Medicine, which advocated for the blinding of researchers.[7] Bernard's recommendation that an experiment's observer should not know the hypothesis being tested contrasted starkly with the prevalentEnlightenment-era attitude that scientific observation can only be objectively valid when undertaken by a well-educated, informed scientist.[8] The first study recorded to have a blinded researcher was conducted in 1907 byW. H. R. Rivers and H. N. Webber to investigate the effects of caffeine.[9] The need to blind researchers became widely recognized in the mid-20th century.[10]
A number of biases are present when a study is insufficiently blinded. Patient-reported outcomes can be different if the patient is not blinded to their treatment.[11] Likewise, failure to blind researchers results inobserver bias.[12] Unblinded data analysts may favor an analysis that supports their existing beliefs (confirmation bias). These biases are typically the result of subconscious influences, and are present even when study participants believe they are not influenced by them.[13]
In medical research, the termssingle-blind,double-blind andtriple-blind are commonly used to describe blinding. These terms describe experiments in which (respectively) one, two, or three parties are blinded to some information. Most often, single-blind studies blind patients to theirtreatment allocation, double-blind studies blind both patients and researchers to treatment allocations, and triple-blinded studies blind patients, researcher, and some other third party (such as a monitoring committee) to treatment allocations. However, the meaning of these terms can vary from study to study.[14]
CONSORT guidelines state that these terms should no longer be used because they are ambiguous. For instance, "double-blind" could mean that the data analysts and patients were blinded; or the patients and outcome assessors were blinded; or the patients and people offering the intervention were blinded, etc. The terms also fail to convey the information that was masked and the amount of unblinding that occurred. It is not sufficient to specify the number of parties that have been blinded. To describe an experiment's blinding, it is necessary to reportwho has been blinded towhat information, andhow well each blind succeeded.[15]
"Unblinding" occurs in a blinded experiment when information becomes available to one from whom it has been masked. In clinical studies, unblinding may occur unintentionally when a patient deduces their treatment group. Unblinding that occurs before the conclusion of anexperiment is a source ofbias. Some degree of premature unblinding is common in blinded experiments.[16] When a blind is imperfect, its success is judged on aspectrum withno blind (or complete failure of blinding) on one end, perfect blinding on the other, and poor or good blinding between. Thus, the common view of studies as blinded or unblinded is an example of afalse dichotomy.[17]
Success of blinding is assessed by questioning study participants about information that has been masked to them (e.g. did the participant receive the drug orplacebo?). In a perfectly blinded experiment, the responses should be consistent with no knowledge of the masked information. However, if unblinding has occurred, the responses will indicate the degree of unblinding. Since unblindingcannot be measured directly, but must be inferred from participants' responses, its measured value will depend on thenature of the questions asked. As a result, it is not possible to measure unblinding in a way that is completely objective. Nonetheless, it is still possible to make informed judgments about the quality of a blind. Poorly blinded studies rank above unblinded studies and below well-blinded studies in thehierarchy of evidence.[18]
Post-study unblinding is the release of masked data upon completion of a study. Inclinical studies, post-study unblinding serves to inform subjects of theirtreatment allocation. Removing a blind upon completion of a study is never mandatory, but is typically performed as a courtesy to study participants. Unblinding that occurs after the conclusion of a study is not a source of bias, because data collection and analysis are both complete at this time.[19]
Premature unblinding is any unblinding that occurs before the conclusion of a study. In contrast with post-study unblinding, premature unblinding is a source of bias. Acode-break procedure dictates when a subject should be unblinded prematurely. A code-break procedure should only allow for unblinding in cases of emergency. Unblinding that occurs in compliance with code-break procedure is strictly documented and reported.[20]
Premature unblinding may also occur when a participant infers from experimental conditions information that has been masked to them. A common cause for unblinding is the presence of side effects (or effects) in the treatment group. In pharmacological trials, premature unblinding can be reduced with the use of anactive placebo, which conceals treatment allocation by ensuring the presence of side effects in both groups.[21] However, side effects are not the only cause of unblinding; any perceptible difference between the treatment and control groups can contribute to premature unblinding.[citation needed]
A problem arises in the assessment of blinding because asking subjects to guess masked information may prompt them to try to infer that information. Researchers speculate that this may contribute to premature unblinding.[22] Furthermore, it has been reported that some subjects of clinical trials attempt to determine if they have received an active treatment by gathering information on social media and message boards. While researchers counsel patients not to use social media to discuss clinical trials, their accounts are not monitored. This behavior is believed to be a source of unblinding.[23] CONSORT standards andgood clinical practice guidelines recommend the reporting of all premature unblinding.[24][25] In practice, unintentional unblinding is rarely reported.[1]
Bias due to poor blinding tends to favor the experimental group, resulting in inflated effect size and risk offalse positives.[24] Success or failure of blinding is rarely reported or measured; it is implicitly assumed that experiments reported as "blind" are truly blind.[1] Critics have pointed out that without assessment and reporting, there is no way to know if a blind succeeded. This shortcoming is especially concerning given that even a small error in blinding can produce astatistically significant result in the absence of any real difference between test groups when a study is sufficientlypowered (i.e. statistical significance is not robust to bias). As such, many statistically significant results inrandomized controlled trials may be caused by error in blinding.[26] Some researchers have called for the mandatory assessment of blinding efficacy in clinical trials.[18]
Blinding is considered essential in medicine,[27] but is often difficult to achieve. For example, it is difficult to compare surgical and non-surgical interventions in blind trials. In some cases,sham surgery may be necessary for the blinding process. A goodclinical protocol ensures that blinding is as effective as possible within ethical and practical constrains.
Studies of blinded pharmacological trials across widely varying domains find evidence of high levels of unblinding. Unblinding has been shown to affect both patients and clinicians. This evidence challenges the common assumption that blinding is highly effective in pharmacological trials. Unblinding has also been documented in clinical trials outside of pharmacology.[28]
A 2018meta-analysis found that assessment of blinding was reported in only 23 out of 408 randomized controlled trials for chronic pain (5.6%). The study concluded upon analysis of pooled data that the overall quality of the blinding was poor, and the blinding was "not successful." Additionally, both pharmaceutical sponsorship and the presence of side effects were associated with lower rates of reporting assessment of blinding.[29]
Studies have found evidence of extensive unblinding inantidepressant trials: at least three-quarters of patients were able to correctly guess their treatment assignment.[30] Unblinding also occurs in clinicians.[31] Better blinding of patients and clinicians reduceseffect size. Researchers concluded that unblinding inflates effect size in antidepressant trials.[32][33][34] Some researchers believe that antidepressants are not effective for the treatment of depression and only outperform placebos due tosystematic error. These researchers argue that antidepressants are justactive placebos.[35][36]
While the possibility of blinded trials onacupuncture is controversial, a 2003 review of 47randomized controlled trials found no fewer than four methods of blinding patients to acupuncture treatment: 1) superficial needling of true acupuncture points, 2) use of acupuncture points which are not indicated for the condition being treated, 3) insertion of needles outside of true acupuncture points, and 4) the use of placebo needles which are designed not to penetrate the skin. The authors concluded that there was "no clear association between type of sham intervention used and the results of the trials."[37]
A 2018 study on acupuncture which used needles that did not penetrate the skin as a sham treatment found that 68% of patients and 83% of acupuncturists correctly identified their group allocation. The authors concluded that the blinding had failed, but that more advanced placebos may someday offer the possibility of well-blinded studies in acupuncture.[38]
It is standard practice in physics to perform blinded data analysis. After data analysis is complete, one is allowed to unblind the data. A prior agreement to publish the data regardless of the results of the analysis may be made to preventpublication bias.[13]
Social science research is particularly prone toobserver bias, so it is important in these fields to properly blind the researchers. In some cases, while blind experiments would be useful, they are impractical or unethical. Blinded data analysis can reduce bias, but is rarely used in social science research.[39]
In apolice photo lineup, an officer shows a group of photos to a witness and asks the witness to identify the individual who committed the crime. Since the officer is typically aware of who the suspect is, they may (subconsciously or consciously) influence the witness to choose the individual that they believe committed the crime. There is a growing movement in law enforcement to move to a blind procedure in which the officer who shows the photos to the witness does not know who the suspect is.[40][41]
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.[42] Blind tests can also be used to compare the quality of musical instruments.[43][44]
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.
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