Although these designs can be used with groups of participants, their particular advantage is that they provide researchers with an option for data collection and interpretation in situations in which a single individual is being treated, observed, and measured.
This option is especially valuable when researchers want to obtain cause-and-effect answers in applied situations.
For example, a clinician would like to demonstrate that a specific treatment actually causes a client to make changes in behavior, or a school psychologist would like to demonstrate that a counseling program really helps a student in academic difficulty.