<P> 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 . </P> <P> A classic example of a single - blind test is the Pepsi Challenge . A tester, often a marketing person, prepares two sets of cups of cola labeled "A" and "B". One set of cups is filled with Pepsi, while the other is filled with Coca - Cola . The tester knows which soda is in which cup but is not supposed to reveal that information to the subjects . Volunteer subjects are encouraged to try the two cups of soda and polled for which ones they prefer . 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). If the tester is a marketing person employed by the company which is producing the challenge, there's always the possibility of a conflict of interest where the marketing person is aware that future income will be based on the results of the test . </P> <P> 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 . Double - blind studies were first used in 1907 by W.H.R. Rivers and H.N. Webber in the investigation of the effects of caffeine . </P> <P> In most cases, double - blind experiments are regarded to achieve a higher standard of scientific rigor than single - blind or non-blind experiments . </P>

In experiments the double-blind technique is used to avoid