The key that identifies the subjects and which group they belonged to is kept by a third party, and is not revealed to the researchers until the study is over.
Double-blind methods can be applied to any experimental situation in which there is a possibility that the results will be affected by conscious or unconscious bias on the part of researchers, participants, or both, for example, in animal studies, both the carer of the animals and the assessor of the results have to be blinded; otherwise the carer might treat control subjects differently and alter the results.
A well-known violinist played each instrument while the committee listened in the next room to avoid prejudice.
One of the first essays advocating a blinded approach to experiments in general came from Claude Bernard in the latter half of the 19th century, who recommended splitting any scientific experiment between the theorist who conceives the experiment and a naive (and preferably uneducated) observer who registers the results without foreknowledge of the theory or hypothesis being tested.
Voting systems are also examples where bias can easily be constructed into an apparently simple machine based system; in analogy to the human researcher described above, the part of the software that provides interaction with the human is presented to the subject as 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.
One of the problems with a single-blind test like this is that the tester can unintentionally give subconscious cues which influence the subjects; in addition, it is possible the tester could intentionally introduce bias by preparing the separate sodas differently (e.g., by putting more ice in one cup or by pushing one cup closer to the subject). In these double-blind experiments, neither the participants nor the researchers know which participants belong to the control group, nor the test group.
Volunteer subjects are encouraged to try the two cups of soda and polled for which ones they prefer. In most cases, double-blind experiments are regarded to achieve a higher standard of scientific rigor than single-blind or non-blind experiments.Double-blind describes an especially stringent way of conducting an experiment which attempts to eliminate subjective, unrecognized biases carried by an experiment's subjects (usually human) and conductors. Performing an experiment in double-blind fashion can greatly lessen the power of preconceived notions or physical cues (e.g., the placebo effect, observer bias, experimenter's bias) to distort the results (by making researchers or participants behave differently from in everyday life).Random assignment of test subjects to the experimental and control groups is a critical part of any double-blind research design.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.Single-blind describes experiments where information that could introduce bias or otherwise skew the result is withheld from the participants, but the experimenter will be in full possession of the facts.A classic example of a single-blind test is the Pepsi Challenge.