AB Design — AB Single-Subject Experimental Design
AB Single-Subject Experimental Design · Also known as: baseline-intervention design, AB single-case design, AB phase design
The AB design is the simplest single-subject experimental design, consisting of two sequential phases: a baseline phase (A) in which the target behavior is observed under natural conditions without intervention, followed by an intervention phase (B) in which the treatment or manipulation is introduced. Changes in the behavior's level, trend, or variability between phases are used to infer the effect of the intervention on the individual participant.
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When to use it
Use the AB design when you need a practical, low-resource single-subject framework to document the association between an intervention and behavior change in an individual — common in applied behavior analysis, special education, clinical psychology, and rehabilitation. It is appropriate when a reversal (withdrawing the treatment) is ethically or practically impossible, or when the goal is preliminary documentation rather than strong experimental control. Do not use the AB design when you need causal evidence: without a return-to-baseline or replication phase, the design cannot rule out history, maturation, or regression to the mean. When internal validity is the priority, upgrade to an ABA or ABAB design.
Strengths & limitations
- Maximally simple to implement — requires only two phases and can be conducted by a single practitioner with one participant.
- Ethically feasible when withdrawing or reversing a presumably beneficial treatment would be inappropriate.
- Provides an individualized picture of behavior change that group-design averages would obscure.
- Serves as the foundational building block from which stronger withdrawal and multiple-baseline designs are constructed.
- Visual analysis of time-series data makes findings interpretable to practitioners without advanced statistical training.
- Weak internal validity: the absence of a withdrawal or replication phase means coincidental events (history) cannot be ruled out as the cause of the observed change.
- Results apply only to the individual studied; generalization to other people, settings, or behaviors requires systematic replication across multiple cases.
- Dependent on a stable, predictable baseline — variable or trending baselines make post-intervention change uninterpretable.
- The design cannot detect delayed treatment effects if behavior change lags behind the phase transition by multiple sessions.
Frequently asked
Why is the AB design considered weak compared to ABA or ABAB designs?
The AB design shows that behavior changed after the intervention was introduced, but it cannot demonstrate that the intervention caused the change. Without withdrawing the treatment and observing whether behavior returns to baseline — as in an ABA design — or replicating that reversal — as in ABAB — alternative explanations such as coincidental events, maturation, or spontaneous recovery cannot be ruled out. Stronger designs provide the within-subject replication that establishes experimental control.
How many data points do I need in each phase?
The conventional minimum is three to five data points per phase, though more is better. The practical criterion is stability: collect data until the level and trend are predictable enough that you can project where data would go if nothing changed. For the intervention phase, continue until the effect or lack of effect is clear and the series has stabilized.
Can I use statistical tests with AB data?
Yes. Visual analysis is the primary and traditional approach, but several statistical supplements are used: Tau-U accounts for baseline trend and provides an effect-size estimate; piecewise regression (interrupted time-series analysis) models level and slope changes at the phase boundary. These complement visual analysis and require at least eight to ten data points per phase to be meaningful.
When is the AB design ethically preferable over a stronger reversal design?
When the intervention is expected to produce irreversible learning (e.g., acquiring a new academic skill) or when withdrawing a beneficial treatment would be harmful or unethical, the reversal required by ABA or ABAB designs is inappropriate. In these situations the AB design — or a multiple-baseline design, which avoids withdrawal entirely — is the ethical choice.
Is the AB design the same as a pre-post design?
No. A pre-post design typically involves a single measurement before and one after an intervention, often across a group. The AB design involves repeated, frequent measurements over time within a single participant, producing a time-series that reveals trends and variability — information a single pre and post measurement cannot provide.
Sources
- Sidman, M. (1960). Tactics of Scientific Research: Evaluating Experimental Data in Psychology. Basic Books. link ↗
- Baer, D. M., Wolf, M. M., & Risley, T. R. (1968). Some current dimensions of applied behavior analysis. Journal of Applied Behavior Analysis, 1(1), 91-97. DOI: 10.1901/jaba.1968.1-91 ↗
How to cite this page
ScholarGate. (2026, June 3). AB Single-Subject Experimental Design. ScholarGate. https://scholargate.app/en/experimental-design/ab-design
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
- ABA DesignExperimental design↔ compare
- ABAB designExperimental design↔ compare
- Alternating Treatments DesignDisability Studies↔ compare
- Changing Criterion DesignDisability Studies↔ compare
- Multiple Baseline DesignExperimental design↔ compare
- Single-Subject Experimental DesignExperimental design↔ compare