Experimental questions are decisive in determining the nature of the experimental design to be selected. There are four basic types of experimental questions: demonstration, comparison, parametric, and component. A demonstration is "Does A cause or influence B?". A comparison is "Does A1 or A2 cause or influence B more?". A parametric question is "How much of A will cause how much change or influence on B?". A component question is "Which part of A - A1 or A2 or A3... - causes or influences B?" where A is composed of parts which can be separated and tested. The A-B-A-B design is useful for demonstration questions.
A-B-A-B
A-B
An AB design is a two part or phase design composed of a baseline with no changes, and a treatment or intervention phase. If there is a change then the treatment may be said to have had an effect. However, it is subject to many possible competing hypotheses, making strong conclusions difficult. Variants on the AB design introduce ways to control for the competing hypotheses to allow for stronger conclusions.
Reversal or A-B-A
The reversal design is the most powerful of the single-subject research designs showing a strong reversal from baseline to treatment and back again. If the variable returns to baseline measure without a treatment then resumes its effects when reapplied, the researcher can have greater confidence in the efficacy of that treatment. However, many interventions cannot be reversed, some for ethical reasons and some for practical reasons. Further ethics notes: It may be unethical to end an experiment on a baseline measure if the treatment is self-sustaining and highly beneficial and/or related to health. Control condition participants may also deserve the benefits of research once all data has been collected. It is a researcher's ethical duty to maximize benefits and to ensure that all participants have access to those benefits when possible.
A-B-C
The A-B-C design is a variant that allows for the extension of research questions around component, parametric and comparative questions.
Multielement
Multi-element designs sometimes referred to as alternating-treatment designs are used in order to ascertain the comparative effect of two treatments. Two treatments are alternated in rapid succession and correlated changes are plotted on a graph to facilitate comparison.
Multiple baseline
The multiple baseline design was first reported in 1960 as used in basic operant research. It was applied in the late 1960s to human experiments in response to practical and ethical issues that arose in withdrawing apparently successful treatments from human subjects. In it two or more behaviors, people or settings are plotted in a staggered graph where a change is made to one, but not the other two, and then to the second, but not the third behavior, person or setting. Differential changes that occur to each behavior, person or in each setting help to strengthen what is essentially an AB design with its problematic competing hypotheses.
Repeated Acquisition
In addition to multiple baseline designs, a way to deal with problematic reversibility is the use of repeated acquisitions.
Brief
A designed favored by applied settings researchers where logistical challenges, time and other limits make research difficult are variants of multi-element and A-B-A-B type designs.
Combined
The combined design has arisen from a need to obtain answers to more complex research questions. Combining two or more single-case designs, such as A-B-A-B and multiple baseline, may produce such answers.
In a changing-criterion research design a criterion for reinforcement is changed across the experiment to demonstrate a functional relation between the reinforcement and the behavior.
