A Single-Subject Design is used when there is one person being studied. There is a phase where no treatment is being offered, this is called the baseline phase. This is followed by a phase that includes treatment. The subject during the baseline phase is the control for the study. In order to assess the effects of the treatment, the dependent variable is measured at regular intervals throughout the study. AB Design has a baseline phase (A) and a treatment phase (B). The dependent variable is measured at regular intervals during both phases. In our example, if a person is asked to stay indoors for five days and go outside in the sun for five days. His depression is then measured every day during both phases.
ABA or ABAB Designs are called reversal or withdrawal designs because you have to withdraw the treatment to bring the person back to the baseline phase (A).
In a Multiple Baseline Design, a treatment is not withdrawn, usually due to the ethical reasons. Treatment can be applied to different tasks by the same subject, different settings the subject is in, or different behaviors portrayed by the same subject. It is considered an AB design since treatment is never withdrawn.