Differential Attrition
The term attrition refers to participant withdrawal from a research study before it is completed. As long as the rate of attrition is fairly consistent from one group to another, it usually is not a threat to internal validity. However, big differences in attrition rates between groups can create problems. The different groups are initially created to be as similar as possible; if large numbers of individuals leave one group, the group may no longer be similar to the other Again, whenever the groups of participants are noticeably different, the research is confounded. Differential attrition refers to differences in attrition rates from one group to another and can threaten the internal validity of a between-subjects experiment.
For example, a researcher may want to test the effectiveness of a dieting program. Using a between-subjects design, the researcher forms two groups of participants with approximately equal characteristics (weight, gender, dieting history). Next, one group of participants begins the 10-week dieting program and the other group receives no treatment (this group, recall from Chapter 9, is the no treatment control ). At the end of the 10 weeks, the weights of the two groups are compared. During the course of the 10 weeks, however, it is likely that some participants will drop out of the study. If more participants drop out of one group than the other, there is a risk that the two groupswill no longer be similar. For example, some of the individuals in the dieting program may decide that it is too demanding and withdraw from the study. As a result, only the most motivated participants stay in the diet program. Although the study started with two equivalent groups, the individuals who are left in the program at the end have a higher level of motivation than those in the control group. In this case, the difference in dropout rate between the groups could account for the obtained differences in mean weight. Differential attrition is a threat to internal validity because we do not know whether the obtained differences between treatment conditions are caused by the treatments or by differential attrition. Whenever participants drop out of a study, a researcher must be concerned about differential attrition as an alternative explanation for treatment effects.