Single-blind ABAB Design — Single-blind Reversal Design
Also known as: single-blind reversal design, single-masked ABAB design, blinded single-case reversal design, single-blind withdrawal design
The single-blind ABAB design is a single-case experimental approach that sequences two baseline phases (A) and two intervention phases (B) to demonstrate experimental control over a target behavior, while keeping one party — typically the outcome assessor or the participant — unaware of current phase assignment. This blinding procedure reduces observer bias or demand characteristics, strengthening the internal validity of the reversal logic.
Key highlights
- Replication within a single participant across two B phases provides strong evidence of experimental control without requiring a large sample.
- Single-blinding of the assessor directly addresses observer bias, improving measurement objectivity relative to an unblinded ABAB design.
- Repeated measurement across time yields rich data on behavioral trajectories and the speed of treatment effects.
- The design is ethically tractable in many clinical settings because it does not permanently withhold treatment — the intervention is reintroduced in B2.
- Applicable when randomized group designs are impractical due to small or unique populations (e.g., rare developmental conditions).
Intuition
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How it works
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When to use it
Use a single-blind ABAB design when you need to establish experimental control over a reversible target behavior in a single participant or a small set of individuals and observer bias is a credible threat — for example, when outcome data are collected through subjective coding of video recordings or behavioral rating scales. It is well suited to applied behavior analysis, clinical psychology, special education, and rehabilitation settings where the target behavior can be reliably measured repeatedly over time and is expected to reverse when treatment is withdrawn. Do not use it when: (a) the target behavior is irreversible after intervention (e.g., a learned skill that cannot be unlearned), because the A2 phase cannot ethically or practically be implemented; (b) withholding treatment in A2 poses an ethical risk to the participant; (c) the behavior is highly variable and unlikely to reach a stable baseline; or (d) full double-blind conditions are required and it is feasible to blind both parties.
Strengths & limitations
- Replication within a single participant across two B phases provides strong evidence of experimental control without requiring a large sample.
- Single-blinding of the assessor directly addresses observer bias, improving measurement objectivity relative to an unblinded ABAB design.
- Repeated measurement across time yields rich data on behavioral trajectories and the speed of treatment effects.
- The design is ethically tractable in many clinical settings because it does not permanently withhold treatment — the intervention is reintroduced in B2.
- Applicable when randomized group designs are impractical due to small or unique populations (e.g., rare developmental conditions).
- Requires behavioral reversibility: if the target behavior does not return toward baseline in A2, experimental control cannot be demonstrated and the logic of the design collapses.
- Single-blind only: observer bias is reduced, but participant awareness of treatment phases (and associated demand effects or reactivity) remains uncontrolled.
- Findings are idiographic — generalization to other individuals requires systematic replication across cases, not statistical inference.
- Frequent phase changes and the associated data-collection burden can be logistically demanding in applied settings.
Common pitfalls
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Applications
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Frequently asked
What exactly is kept blind in a single-blind ABAB design?
In the most common implementation, the outcome assessor — the person who measures or codes the target behavior — is kept unaware of which phase (A baseline or B intervention) is currently active. Alternatively, in some designs the participant is kept unaware (e.g., when the intervention is an unannounced environmental change). Only one party is blinded; if both are blinded it becomes a double-blind ABAB design.
What if the behavior does not reverse in the A2 phase?
Non-reversal in A2 means experimental control cannot be demonstrated using this design. It may indicate that the behavior change is due to learning or other irreversible processes rather than the ongoing intervention. In that case, a multiple baseline design — which does not require withdrawal — is a more appropriate alternative.
How do I maintain blinding when the intervention involves visible procedures?
This is the central practical challenge. Strategies include: using pre-recorded sessions coded later by raters who receive only anonymized files, removing date and session-order information from observation forms, standardizing the room and observer behavior across all phases, and training raters on coded rather than labeled phase materials.
How many data points are needed per phase?
There is no absolute minimum, but most methodologists recommend at least three to five data points per phase, with stability criteria met before transitioning. More data points per phase increase confidence in the trend and level estimates used during visual analysis. Kazdin (2011) and Barlow et al. (2009) both provide detailed guidance on stability criteria.
Can this design be used with multiple participants simultaneously?
Yes. Running the same single-blind ABAB design with several participants in a replicated series strengthens external validity considerably, though the phases need not be staggered as in a multiple baseline design. Each participant's data are analyzed individually; cross-participant consistency is then examined as evidence of generality.
Sources
- 1.Kazdin, A. E. (2011). Single-Case Research Designs: Methods for Clinical and Applied Settings (2nd ed.). Oxford University Press.ISBN 978-0195341881
- 2.Barlow, D. H., Nock, M. K., & Hersen, M. (2009). Single Case Experimental Designs: Strategies for Studying Behavior Change (3rd ed.). Pearson.ISBN 978-0205474554
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ScholarGate. (2026, June 3). Single-blind ABAB design. ScholarGate. https://scholargate.app/experimental-design/single-blind-abab-design