Blinded experiment
In a blind or blinded experiment, information which may influence the participants of the experiment is withheld until after the experiment is complete. Good blinding can reduce or eliminate experimental biases 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 good clinical protocol ensures that blinding is as effective as possible within ethical and practical constraints.
During the course of an experiment, a participant becomes unblinded 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 on pain medication and antidepressants 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. The CONSORT reporting guidelines recommend that all studies assess and report unblinding. In practice, very few studies do so.[1]
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] In clinical research, a trial that is not a blinded trial is called an open trial.
History[edit]
The first known blind experiment was conducted by the French Royal Commission on Animal Magnetism in 1784 to investigate the claims of mesmerism as proposed by Charles d'Eslon, a former associate of Franz 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.
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]
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 of Friedrich Schiller.[6] This trial contested the effectiveness of homeopathic dilution.[5]
In 1865, Claude Bernard published his Introduction 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 prevalent Enlightenment-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 by W. 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]
Background[edit]
Bias[edit]
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 in observer 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]
Applications[edit]
In medicine[edit]
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 good clinical 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]