In a single-subject design, researchers typically evaluate the final impact of an experimental intervention immediately upon entering the treatment phase, regardless of initial fluctuations in the dependent variable.

Following the establishment of a baseline in a single-subject design, the treatment phase (Phase B) is initiated by introducing the experimental intervention. The dependent variable may fluctuate initially as the participant adjusts, so the researcher waits until responding reaches a new steady state to accurately assess the treatment's impact.

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The reversal design, also referred to as the ABA design, is the most fundamental single-subject research design. In this approach, a researcher first establishes a baseline of the dependent variable (Phase A), then introduces a treatment (Phase B), and finally removes the treatment to return the participant to the baseline condition (Phase A).

Reversal Design

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In a single-subject reversal design, the initial step is establishing a baseline for the dependent variable during Phase A. This phase acts as a control condition, representing the natural level of responding before any intervention is applied, and continues until a steady state of behavior is achieved.

Baseline Phase

Treatment Phase

The removal of the treatment in a reversal design significantly increases a study's internal validity compared to a simpler AB design. By demonstrating that the dependent variable changes when the treatment is introduced and subsequently reverts when the treatment is withdrawn—assuming the treatment does not create a permanent effect—researchers can more confidently rule out extraneous variables and conclude that the intervention caused the observed changes.

Internal Validity of Reversal Designs

A multiple-treatment reversal design expands upon the basic reversal structure to evaluate the effectiveness of more than one intervention. In this approach, an initial baseline phase is followed by separate phases for distinct treatments, often returning to a baseline between treatments to control for carryover effects. For example, a researcher evaluating a disruptive student might establish a baseline of studying behavior (A), introduce positive teacher attention (B), and then switch to mild punishment for not studying (C), potentially creating an ABCACB sequence.

Multiple-Treatment Reversal Design

An alternating treatments design is a variation of single-subject research where two or more distinct treatments are rapidly alternated on a regular schedule. For example, positive attention for studying could be used one day and mild punishment for not studying the next, or one treatment could be applied in the morning and another in the afternoon. This design is a quick and effective way to compare interventions, but only when the treatments are fast-acting.

Alternating Treatments Design

An ABAB design is an extension of the basic reversal design that involves an initial baseline phase (A), a treatment phase (B), a return to baseline through the removal of the treatment (A), and a second reintroduction of the treatment (B). For example, Hall and his colleagues utilized an ABAB design to demonstrate that a student's studying behavior increased when positive attention was provided, decreased when it was removed, and increased again when it was reintroduced.

ABAB Design

The removal of the treatment in a reversal design is essential for demonstrating causality. While a simple AB design cannot easily rule out the possibility that an extraneous variable caused the change in the dependent variable, a reversal design addresses this issue. If the dependent variable changes when the treatment is introduced and then reverts when the treatment is removed, it strongly indicates that the treatment caused the effect. This reversal process significantly improves the internal validity of the study, provided the treatment's effects are not permanent.

Internal Validity in Reversal Designs

An ABAB reversal design can be illustrated by a study aimed at improving a student's studying behavior. The percentage of time spent studying is measured and found to be low during the initial baseline phase. A treatment is then introduced, causing studying time to increase until it stabilizes. When the treatment is removed during the second baseline phase, the studying behavior decreases. Finally, the treatment is reintroduced in a second treatment phase, which leads to studying behavior increasing once again.

ABAB Reversal Design Example

The reversal design has two potential problems related to the removal of treatment. First, removing a treatment that is working can be unethical, such as taking away an intervention that reduces self-injury. Second, the dependent variable may not return to baseline once the treatment is removed, making it difficult to determine whether the treatment caused the change or if another factor was responsible.

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