10.6 APPLICATIONS AND STATISTICAL ANALYSES OF BETWEEN-SUBJECTS DESIGNS
Two-Group Mean Difference
The simplest of a between-subjects experimental design involves comparing only two groups of participants: the researcher manipulates one independent variable with only two levels. This design is often referred to as the single-factor two-group design or simply the two-group design. This type of design can be used to compare treatments, or to evaluate the effect of one treatment by comparing a treatment group and a control group. When the measurements consist of numerical scores, typically, a mean is computed for each group of participants, and then an independent-measures t tent is used to determine whether there is a significant difference between the means (sec Chapter 14).
The primary advantage of a two-group design is its simplicity. It is easy to set up a two-group study, and there is no subtlety or complexity when interpreting the results; either the two groups are different or they are not. In addition, a two-group design provides the best opportunity to maximize the difference between the two treatment conditions; that is, you may select opposite extreme values for the independent variable. For example, in a study comparing two types of therapy, the two therapies con be structured to maximize or even exaggerate the differences between them. Or, in a research study comparing a treatment and a no-treatment control, the treatment group can be given the full-strength version of the treatment. This technique increases the likelihood of obtaining noticeably different scores from the two groups, thereby demonstrating a significant mean difference.
The primary disadvantage of a two-group design is that it provides relatively little information. With only two groups, a researcher obtains only two real data points for comparison. Although two data points are sufficient to establish a difference, they often are not sufficient to provide a complete or detailed picture of the full relationship between an independent and a dependent variable. Figure 10.5 shows a hypothetical relationship between dosage levels for a drug (independent variable). Notice that the complete set of five data points, representing five different drug doses, gives a good picture of how drug dosage affects. Now, consider the limited data that would be available if the researcher had used only two different drug doses. If, for example, the researcher had used only a 0-dose and