Figure 9.1. Design of the classical experiment

Testing hypotheses with a control group

Generally a hypothesis calls for a larger difference between pretest and posttest results for the experimental than for the control group. The basis for this reasoning is that:

1. The two groups were alike with respect to some dependent variable at the beginning of the experiment.
2. Subjects in both groups were exposed to the same kinds and amounts of extraneous or outside influences during the course of the experiment.
3. Subjects in the experimental group, however, were exposed to an additional and deliberately applied influence, the education program, which was designed to cause a reduction in prejudice.
4. The experimental group, therefore, would be expected to show a greater decline in prejudice.

Use of the control group allows us to detect the effects of the experimental variable on levels of prejudice. If the posttest measure shows about the same reduction in both the experimental and control groups, then the change must be due to some external factor. But, if prejudice levels decline substantially only in the experimental group, it is reasonable to conclude that the decline was caused by the treatment we administered. Even if prejudice scores in both groups declined, but more so in the experimental group, we could take this as evidence of the effect of the educational program. In more precise terms, the difference in the pretest and posttest measurements for the experimental versus the control group is known as the experimental effect. This comparison, however, assumes that the subjects in two groups were equivalent at the beginning of the experiment. And this depends on how the subjects were assigned to each group.