In experimental research, it is important to confirm that results of a study are actually due to an independent or manipulated variable rather than to other, extraneous variables. In the simplest case, a research study contrasts two groups, and the independent variable is present in one group but not the other. For example, in a health research study, one group may receive a medical treatment, and the other does not. The first group, in which treatment occurs, is called the experimental group, and the second group, in which treatment is withheld, is called the control group. Therefore, when experimental studies use control and experimental groups, ideally the groups are equal on all factors except the independent variable. The purpose, then, of a control group is ...