Читать книгу Introduction to Abnormal Child and Adolescent Psychology - Robert Weis - Страница 137

A nonequivalent groups study is a more sophisticated quasi-experimental study in which researchers compare participants in treatment and control groups, but participants are not randomly assigned to these groups. Because the researchers do not randomly assign participants to the two groups, the groups are nonequivalent—that is, they may differ in some way before treatment. Therefore, if the researchers notice differences between groups after treatment, they cannot definitively say that treatment caused those differences.

For example, researchers wanted to see if medication for ADHD in childhood might reduce the likelihood of alcohol and drug use problems in adolescence. Ideally, the researchers would randomly assign children to either a treatment group that receives medication or to a control group that does not. Then, several years later, they could compare the prevalence of substance use problems among adolescents in the two groups.

However, the researchers could not randomly assign children to treatment and control groups because it would be unethical to delay treatment to children with ADHD for so long. Instead, the researchers compared a group of adolescents with ADHD who took medication for this condition as children to another group of adolescents with ADHD who had no history of using medication to manage their symptoms. The researchers found that youths with ADHD who took medication were less likely to use alcohol and other drugs than youths with ADHD who never used medication (Hammerness, Petty, Faraone, & Biederman, 2017).

Because their study included a control group, there are fewer threats to its internal validity. For example, it is unlikely that maturation explains differences in adolescents’ substance use because both groups of adolescents were assessed at the same point in their development. Similarly, the dropout rate of participants in both groups was roughly equal, ruling out attrition as a possible explanation for their findings. However, the use of nonequivalent groups can introduce a special threat to internal validity called selection bias.

Selection bias refers to a systematic difference between the treatment and control groups that emerges when groups are not randomly assigned. Because their study lacked random assignment, youths in the treatment and control groups were nonequivalent at the beginning of the study. It is possible that subtle differences in demographic variables, family background, or attitudes toward medication existed between the two groups at the beginning of the study. Therefore, the differences in substance use seen at the end of the study might be explained by these subtle differences rather than the medication itself.