Experimental Group

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 there are subliminal (i.e., undetectable) messages embedded in the tapes that have an unconscious influence on the listeners’ motivations. Consequently, someone wishing to stop smoking could listen to such a tape on a regular basis, with the expectation that smoking frequency would decline after a few weeks.

To test these products, a research psychologist would conduct a controlled experiment. Research participants would be recruited, and perhaps even paid to participate in the study. Most smokers report difficulty in quitting, even when motivated to do so, thus finding interested volunteers would not be too difficult. Half the participants would be assigned to the “experimental group,” and the other half to the “control group.” Subjects in the experimental group would be provided with subliminal tapes designed to assist in smoking cessation. Those in the control condition would receive identical tapes, except that the control tapes would contain no subliminal messages. This condition is sometimes referred to as the “placebo” condition.

The purpose of randomly assigning subjects to the experimental and control conditions is to try to insure that the two groups are roughly equivalent with respect to characteristics that could affect their reactions to the tapes. For example, the groups should not differ in terms of the composition of heavy versus light smokers. Nor should there be highly motivated individuals in one group, and relatively indifferent participants in the other such as might happen if subjects were assigned to groups in the order that they called in to volunteer for the study. Random assignment makes it unlikely that groups differ much from one another on such factors. Establishing the equivalence of groups beforehand is important because the researchers are predicting that people in the experimental group will either stop smoking, or will smoke less after receiving the treatment compared to the control group (assuming that the treatment works). A difference in cessation rates between the two groups will be interpreted as evidence of the efficacy of the tapes, as long as the groups are equivalent beforehand. If the groups differed from one another beforehand (say in terms of motivation), it would be impossible to attribute subsequently observed differences to the treatment.

Some additional factors would also need to be controlled. Expectancy effects can affect peoples' behaviors, independently of whatever treatment they might be receiving. Participants in the experimental group, for example, might expect their smoking to decline because of their faith in the tapes and the knowledge about the subliminal messages contained on them. They could become more self-conscious about their smoking habits, and consequently smoke less. If such a change were to occur, it could be a result of the subjects' beliefs and expectations about the treatment, rather than the treatment itself. Expectancy effects can be so strong beliefs about treatment have even been found to cause reductions in perceived pain when subjects believed they were given pain relief medication but were actually given sugar pills (also called a placebo effect).

The experiment described is relatively simple, but it contains the essential properties of any true experiment—random assignment of subjects to conditions, neither subject or researcher knowing which group subjects are in, and the use of a control group. More complex experimental designs may consist of several different conditions or treatments, along with relatively elaborate controls. Controlled experiments are a powerful means of discovering cause-effect relationships, but other research techniques are also valuable. Sometimes experimentation is impossible because of practical or ethical constraints. For example, questions about the effects of homelessness, neglect, or malnutrition cannot be investigated by experimental means. The careful use of alternative methods can help answer some of these difficult yet important questions.



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Schneider, Sandra. Experimental Design in the Behavioral and Social Sciences. Los Angeles: SAGE, 2013.