Research on the treatment of challenging behaviors such as aggression, tantrums, and self-injury expanded significantly over the past two decades. However, despite of the rather impressive numbers of studies, it is still uncertain whether positive behavioral support (PBS) is effective with everyone. To be able to tell family members and professionals that procedures are effective, the research needs to meet certain standards of proof. One of those standards involves a clear idea about the population examined and the conditions under which treatment is successful. In this article, the authors attempt to answer the following questions: How do researchers in the field of PBS select the participants for their research studies? Do participants drop out of PBS prematurely, and if so, who leaves and who remains? This seemingly small step in the research process could have a major impact on how one interprets the outcomes. As such, the authors suggest that concerns about selection of participants must be addressed in all clinical outcome studies.