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.
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.
A classic example of a single-blind test is the “Pepsi challenge”. A marketing person prepares several cups of cola labeled “A” and “B”. One set of cups has Pepsi, the others have Coca-Cola. The marketing person 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. The problem with a single-blind test like this is the marketing person can give subconscious cues which bias the volunteer, whether or not these were intended. In addition it is possible the marketing person could prepare the separate sodas differently (more ice in one cup, push one cup in front of the volunteer, etc.) which can cause a bias. If the marketing person is employed by the company which is producing the challenge there’s always the possibility of a conflict of interests where the marketing person is aware that future income will be based on the results of the test.