Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
The double blind method is an important part of the scientific method, used to prevent research outcomes from being 'influenced' by either the placebo effect or the observer bias. Blinded research is an important tool in many fields of research, from medicine, to psychology and the social sciences, to forensics.
Blinding is a basic tool to prevent conscious and unconscious bias in research. For example, in open taste tests comparing different product brands, consumers usually choose their regular brand. However, in blind taste tests, where the brand identities are concealed, consumers may favor a different brand.
Single blind describes experiments wherein information that could introduce bias or otherwise skew the result is withheld from the participants. Unlike in a double blind experiment, the experimenter will be in full possession of the facts. A trivial example of a single-blind experiment would be the Pepsi Challenge.
In a single blind experiment, the individual subjects do not know whether they are so-called "test" subjects or members of an "experimental control" group. Single-blind experimental design is used where the experimenters either must know the full facts (for example, when comparing sham to real surgery) and so the experimenters cannot themselves be blind, or where the experimenters will not introduce further bias and so the experimenters need not be blind. However, there is a risk that subjects are influenced by interaction with the researchers — known as the experimenter's bias. Single-blind trials are especially risky in psychology and social science research, where the experimenter has an expectation of what the outcome should be, and may consciously or subconsciously influence the behavior of the subject.
Double-blind describes an especially stringent way of conducting an experiment, usually on human subjects, in an attempt to eliminate subjective bias on the part of both experimental subjects and the experimenters. In most cases, double-blind experiments are held to achieve a higher standard of scientific rigour.
In a double-blind experiment, neither the individuals nor the researchers know who belongs to the control group and the experimental group. Only after all the data are recorded (and in some cases, analyzed) do the researchers learn which individuals are which. Performing an experiment in double-blind fashion is a way to lessen the influence of the prejudices and unintentional physical cues on the results (the placebo effect, observer bias, and experimenter's bias). Random assignment of the subject to the experimental or control group is a critical part of double-blind research design. The key that identifies the subjects and which group they belonged to is kept by a third party and not given to the researchers until the study is over.
Double-blind methods can be applied to any experimental situation where there is the possibility that the results will be affected by conscious or unconscious bias on the part of the experimenter.
Computer-controlled experiments are sometimes also referred to as double-blind experiments, since software should not cause any bias. In analogy to the above, the part of the software that provides interaction with the human is the blinded researcher, while the part of the software that defines the key is the third party. An example is the ABX test, where the human subject has to identify an unknown stimulus X as being either A or B.
Triple-blind trials are double-blind trials in which the statistician interpreting the results also does not know which intervention has been given. Sometimes triple-blind is used to mean that multiple investigators are all blinded to the protocol (such as the clinician giving the treatment and a radiologist or pathologist who interprets the results.) The use of the term triple-blind experiments is disputed.
Double-blinding is relatively easy to achieve in drug studies, by formulating the investigational drug and the control (either a placebo or an established drug) to have identical appearance (color, taste, etc.). Patients are randomly assigned to the control or experimental group and given random numbers by a study coordinator, who also encodes the drugs with matching random numbers. Neither the patients nor the researchers monitoring the outcome know which patient is receiving which treatment, until the study is over and the random code is broken.
Effective blinding can be difficult to achieve where the treatment is notably effective (indeed, studies have been suspended in cases where the tested drug combinations were so effective that it was deemed unethical to continue withholding the findings from the control group, and the general population), or where the treatment is very distinctive in taste or has unusual side-effects that allow the researcher and/or the subject to guess which group they were assigned to. It is also difficult to use the double blind method to compare surgical and non-surgical interventions (although sham surgery, involving a simple incision, might be ethically permitted). A good clinical protocol will foresee these potential problems to ensure blinding is as effective as possible.
Evidence-based medicine practitioners prefer blinded randomised controlled trials (RCTs), where that is a possible experimental design. These are high on the hierarchy of evidence; only a meta analysis of several well designed RCTs is considered more reliable.
In a police photo lineup, an officer shows a group of photos to a witness or crime victim and asks him to pick out the suspect. This is basically a single-blind test of the witness' 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.  
- Experimental control
- Randomized controlled trial (also known as triple blind)
- Clinical trial
- http://skepdic.com/control.html More on why double blind is important.
- PharmaSchool JargonBuster Clinical Trial Terminology Dictionary
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