In an experiment that focuses on the effects of a single condition or variable, the group that is exposed to all the conditions or variables except the one being studied.
Scientists often study how a particular condition or factor influences an outcome. In such an experiment, in which there are two groups of subjects, the group that is exposed to the condition or factor is called the experimental group. The other group, which provides a basis for comparison, is called the control group. For example, in a hypothetical study of the influence of the presence of loud music on the test performances of children, the control group would consist of the group of children not exposed to the loud music during the test. Their test scores would be compared with the experimental group, the group of children who were exposed to loud music during the test. In this type of experimental design, subjects would be randomly assigned to each group to ensure a reliable comparison.
Atkinson, Rita L.; Richard C. Atkinson; Edward E. Smith; and Ernest R. Hilgard. Introduction to Psychology. 9th ed. San Diego: Harcourt Brace Jovanovich, 1987.
Zimbardo, Philip G. Psychology and Life. 12th ed. Glenview, IL: Scott, Foresman, 1988.