A Single-subject design or a single-case design is a type of experimental research concentrated in the study of just one participant. This kind of design ensures better control over the experiment and consequently higher confidence in cause-and-effect and inferences about the variables.
A phase represents the basic structure of the major part of single-subject kinds of experiments, and a phase change means that a change of condition that can happen during the treatment. The goal is to demonstrate that this change occurs followed by a variation of behavior too, that could be observed when we interrupt a therapy, for example. Generally, a phase change is determined by the participant’s responses and the researcher intervention to address any possible problem appropriately.
The phase is essential to establish how the pattern of behavior present in a baseline phase changes when the researcher switches to a treatment phase. This informations can be well presented through a graph; to allow a better interpretation of the differences and the variables’action. Although the evaluation of the graphs is subjective, there are guidelines to introduce some critical aspects that require more focus on the interpretations.
The most usual kind of single-subject design shows that treatment does have an effect. The ABAB design as also known as reversal design, intersperses 4 phases between baseline (A) and treatment condition (B), to demonstrate that the treatment causes a change of participants' behavior. The graphs can provide pieces of evidence of cause-and-effect relationship all the time that variations of the treatment occur leading to a change of levels. The credibility of this design is directly related to what happens when returning to a baseline phase and withdrawing of the treatment in the middle of the process can create serious ethical and practical problems in the success of the study. The ABAB design allows some variations to address some modifications during the development of the study, creating an unlimited number of potential phase sequences to reach an unambiguous result.
In some cases, in the absence of expected results, the first treatment can be modified by creating a new phase, then can again present the same problem. It will be necessary to start a different treatment one more to find effective results of cause-and-effect to qualify the study as an experimental one. In the graph, it will be possible to identify that, the last treatment, in fact, is responsible for the behavior change. Although, researchers have to take into account the possibility of any confounding variable acting with the treatment or, that the efficacy of it could be attributed to the delay of the previous treatments as well. The last consideration is to evaluate if the change of behavior could be the provoked by all treatments together. It is up to the researcher, to make a decision of what to do in situations like that. Among the options, he can start a second baseline in an attempt to recover the first baseline and repeat just the last treatment to verify that the replication of the original effects will occur again. The other possibility is to evaluate the potential of delayed effect or the catalyst effect, going back to the baseline to verify wich sequence of treatments is a necessary requirement for the efficacy for the last treatment.
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