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A group of subjects in a research experiment that receives an experimental treatment.

Psychologists conduct experiments in order to isolate causes and effects. Ultimately, explaining human behavior consists of identifying the factors that have a causal influence on how we think or act. The most effective way to investigate causation is through experimentation. Different aspects of an experiment are best explained by providing an example. Suppose a researcher wants to find out if subliminal, auditory self-help tapes have any therapeutic benefits. These cassette tapes are available by mail order from a number of companies. They purport to help people change all sorts of bad habits, such as over-eating or smoking. Most tapes consist of music, ocean waves, and the occasional bird cry. According to the manufacturers, however, there are subliminal (i.e., undetectable) messages embedded in the tapes that have...