Double-blind describes an especially stringent way of conducting an experiment, usually on living, conscious, human subjects. A blind experiment is designed so that individuals do not know whether they are so-called "test" subjects or members of an "experimental control" group.
In a double-blind experiment, neither the individuals nor the researchers know who belongs to the control group. Only after all the data are recorded (and in some cases, analyzed) may researchers be permitted to learn which individuals are which. Performing an experiment in double-blind fashion is a way to lessen the influence of prejudices and unintentional physical cues on the results.
Strictly speaking, every researcher who interacts with or treats a subject should be blinded, if an experiment is to be designated "double-blind." This situation is easily arranged when the treatment being tested is a drug, the appearance of which may be simulated with a colored pill or solution (that is, a placebo). But with surgical procedures, for example, a surgeon inevitably knows whether it is the procedure or a sham that he or she is performing. The evaluation of such procedures can be approximately double-blind if the researchers responsible for recording subjects' responses and analyzing the data are blinded. Such a test typically is not considered "double-blind."