Can I make a claim that a treatment causes a client to improve?

• Other explanations for improvement exist

Although it seems easy to determine a treatment’s effectiveness, many alternative explanations can frustrate the psychologist’s efforts to claim that the treatment changed the impulsive behavior. For example, the child’s teacher may work with the child using a different treatment (e.g., rewarding thoughtful behavior), or the child may stop behaving impulsively because other children stop playing with him or her. Any of these other "treatments," rather than the psychologist’s treatment, may cause the improved behavior. Single-case research designs require that the psychologist control as many aspects of the treatment situation as possible in order to test the effectiveness of the treatment.

To summarize, if your research question seeks to describe, explain, predict, and/or treat the behavior and mental processes of one individual, you should choose a single-case design.