Читать книгу Introduction to Abnormal Child and Adolescent Psychology - Robert Weis - Страница 136

A pretest-posttest study is a quasi-experimental study in which the same group of participants is measured at least twice: at baseline (before treatment) and at the end of the study (after treatment). Because all participants receive treatment, there is no control group and, therefore, random assignment is not possible.

For example, researchers conducted a pretest-posttest study investigating the effects of stimulant medication on children’s ADHD symptoms. They administered stimulant medication to a large sample of children with ADHD for approximately 12 weeks. To assess outcomes, they asked clinicians, parents, and teachers to rate children’s ADHD symptoms at the beginning and end of the study. Overall, 75% of children showed a significant decrease in symptoms (Döpfner, Görtz-Dorten, Breuer, & Rothenberger, 2011).

Because the study lacked a control group, the researchers could not conclude that the medication caused this reduction in children’s symptoms. It is possible that other factors, besides medication, might better explain the results of this study. Internal validity refers to the degree to which we can say that manipulation of an independent variable (e.g., treatment) causes a corresponding change in a dependent variable (e.g., children’s outcomes). When other factors can explain children’s outcomes, researchers say these factors threaten the internal validity of the study. There are several threats to internal validity that limit the causal inferences we can make from quasi-experimental research (Kazdin, 2017).

First, maturation can compromise the internal validity of a study. Maturation refers to changes in the child that occur because of the passage of time. For example, as children’s brains mature, they show greater capacity for attention, concentration, and impulse control. It is possible that all children, even those who do not receive treatment, will show a reduction in ADHD symptoms simply due to this brain maturation. Unless researchers compare children who receive treatment with children in a control group who do not, the effects of treatment cannot be distinguished from maturation alone.

Second, environmental factors can threaten the internal validity of pretest-posttest studies. Environmental factors include changes in the child’s family (e.g., divorce), school (e.g., a new teacher), or peer group (e.g., best friend moves away). Environmental factors also include major events (e.g., an economic downturn, the COVID-19 pandemic) or more subtle changes in the child’s surroundings. For example, if researchers assessed children’s symptoms at pretest during the school year and at posttest during the summer, parents might report fewer attention problems over time. However, this apparent improvement in attention might be explained by the fact that inattention is less problematic during summer vacation than during the academic year. Without a control group for comparison, it is possible that these environmental changes might explain some of the study’s results.

A third threat to internal validity is repeated testing. The act of repeatedly assessing children can cause them to show improvement over time. For example, if children know that their parents and teachers are monitoring their behavior, they might try to act more attentive or obedient. Similarly, if parents and teachers are repeatedly asked to rate children’s behavior, they might pay more attention to signs of improvement. Without a comparison group, it is possible that some of the benefits of treatment are simply due to the fact that children were monitored and tested multiple times.

Fourth, attrition can threaten the internal validity of a study. Attrition refers to the loss of participants over the course of the study. Attrition usually occurs because participants decide to withdraw from the study or simply stop attending treatment sessions. When a large percentage of participants in the treatment group withdraw from the study, it threatens the study’s internal validity. For example, Döpfner and colleagues (2011) found that 75% of children who completed their study showed a significant reduction in ADHD symptoms. However, this percentage likely overestimates the actual number of children who benefited from the medication. Closer inspection of the data showed that 6% of children in the original sample withdrew from the study prematurely, perhaps because the medication was not effective for them or because it produced unpleasant side effects. Some of the apparent benefits of the medication are partially explained by the fact that these children withdrew from the study.