ABSTRACT
True experimental designs involve random assignment to treatment conditions and are excellent for exploring cause-and-effect relationships. Quasi-experimental designs, which are of intermediate value for exploring cause and effect, should be used. Design 7 is a widely used, quasi-experimental design that has two intact groups. This is the non-equivalent control group design. Even though participants are not assigned to groups at random in Design 7, researchers often use matching to increase the internal validity of the results. The participants in Design 8 are, in effect, serving as both the control participants and the experimental participants. This is a major advantage because researchers know that the experimental participants and control participants are identical—in fact, in all ways—at the beginning of the experiment. In psychology, a single-group design in which treatments are alternated is often called an ABAB design, where A and B indicate alternating treatments. Typically, when this design is used, there are multiple initial observations before any treatment is given.
