Quantitative Single-Case Research Design

Conceptual Overview and Discussion

Theory-building in the quantitative single-case research design is a systematic process implemented with a high degree of researcher–subject matter contact. Instead of creating an a priori theory of human behavior, experiments are conducted to explore more open-ended questions than those that simply seek to determine the demonstrated effects of a treatment. Questions can also evoke comparison across treatments, identification of parametric or incremental changes in the treatments, and multicomponent analysis. Such rich questions can be posed and lead to data sources that have the potential to allow researchers to understand the behavioral processes that cause a technique to be effective.

Montrose Wolf, Donald Baer, and Todd Risely, founding editors of the Journal of Applied Behavioral Analysis, identified seven dimensions of applied behavioral analysis. These seven dimensions articulate the scientific procedures and practices of quantitative single-case research designs.

The first three dimensions deal with the process of completing the single-case study. The first dimension accentuates the area of research focus, which requires the researcher to identify a target behavior to be improved. The second dimension requires that the target behavior or variable be operationalized in terms of physical events or [Page 765]material existence in order to support precision in measurement strategies. Operationalizing a variable requires that the researcher describe the variable in disambiguated terms that set the criteria for a discernable effect on the variable. This allows the researcher to define and rationalize the expected change in behavior before an experimental intervention begins to ensure that treatment goals are met. The third dimension highlights the need for the experiment to demonstrate change in behavior by analytically evaluating the effects of the treatment or intervention on the behavior. Demonstrating this experimental control means establishing a functional relation, which indicates that the dependent variable (the measure of the target behavior) has been influenced by the independent variable (intervention) despite the presence of extraneous factors.

The fourth dimension accentuates the importance of establishing the functional relation through the use of design tactics that specify patterns of intervention using an alphabetic notation, or the “ABCs” of QSCRD. An A phase represents the baseline measurement of the behavior, that is, the behavior in the absence of an intervention. The B phase in QSCRD is the intervention or independent variable. An A-B-A-B design tactic would signify the following sequence of conditions: baseline, intervention, baseline, intervention. A general rule for determining the durations of the phases is to wait until the measure of the behavior is quiescent (stable or consistent over time). Ethical design tactics provide the researcher with a consistent and believable pattern while also not tiring the participant with excessive repetition of the same intervention patterns. Thus, for example, an A-B-A-B-A-B design repeats the same pattern of interventions more often than necessary to discern a functional relation. When an intervention is already in place, a B-A-B design can be used. QSCRD design tactics generally rely on dependent variable sensitivity to the withdrawal or reversibility of the independent variable. At some point in the design tactic, Intervention B is applied, then it is removed and the effect on the participant of reverting to the baseline, A, is noted. Then Intervention B is inserted once more to note its effect on the participant. Additional independent variables can be tested by assigning more alphabetic symbols. For example, the design tactic of a study involving two independent variables, B and C, could be expressed A-B-A-C-A-B-A-C, which would signify baseline, Intervention 1, baseline, Intervention 2, baseline, Intervention 1, baseline, Intervention 2. Interventions can also be combined without an interleaving baseline. A plus sign (+) is used to denote the union of the two interventions in the design tactic; for example, the tactic A-B-A-C-A-B+C-A-B identifies the effect of B, the effect of C, and the effect of B+C and then replicates the effect of B. Furthermore, because the effect of B is replicated one can deduce the replication of C from the effect of B+C, and one can deduce the replication of B+C from the effect of C. Therefore, this is a more ethical pattern than A-B-A-C-A-B+C-A-B-A-C-A-B+C.

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