Why Single-Subject Research Designs Matter
Single-subject research designs (also called single-case experimental designs) are the backbone of applied behavior analysis. Unlike group designs used in many other fields, ABA relies on individual analysis because behavior change is evaluated at the individual level. Each client serves as their own control, allowing behavior analysts to draw conclusions about the effects of interventions on specific people in specific contexts.
The BCBA exam tests your ability to identify, compare, and apply these designs. You need to know not just what each design looks like, but when to choose one over another based on the clinical situation, the behavior being targeted, and ethical considerations.
The Logic of Single-Subject Designs
All single-subject designs share a common logic: they demonstrate experimental control by showing that changes in behavior occur when (and only when) the independent variable is introduced. This is accomplished through prediction, verification, and replication. The baseline phase establishes a pattern of responding that allows the researcher to predict what would happen without intervention. Verification occurs when the behavior changes upon introduction of the independent variable. Replication strengthens the conclusion by demonstrating the effect multiple times.
Reversal Design (ABAB)
The reversal design is one of the most straightforward and powerful single-subject designs. It involves alternating between baseline (A) and treatment (B) conditions. A basic reversal design includes at least four phases: baseline, treatment, return to baseline, and treatment again.
When to use it: The reversal design works best when the target behavior is expected to reverse (return to baseline levels) when the intervention is withdrawn. It is ideal for behaviors maintained by environmental contingencies that can be temporarily removed.
Strengths: It provides a strong demonstration of experimental control because the behavior changes in both directions. It is relatively easy to implement and interpret.
Limitations: It is not appropriate when the behavior is unlikely to reverse (such as academic skills that, once learned, are retained) or when reversing the behavior would be unethical (such as withdrawing treatment for dangerous self-injury).
Multiple Baseline Design
Multiple baseline designs demonstrate experimental control by introducing the intervention in a staggered fashion across two or more baselines. There are three primary variations: across behaviors, across settings, and across participants.
Across behaviors: The intervention is applied to different behaviors of the same individual at different points in time. Each behavior should be functionally independent so that treating one does not affect the others.
Across settings: The same behavior is measured in multiple settings, and the intervention is introduced in each setting at different times.
Across participants: The same behavior and intervention are studied with different individuals, with treatment introduced at staggered intervals.
When to use it: Multiple baseline designs are ideal when reversal is not feasible or ethical. They require no withdrawal of treatment, making them a practical choice in clinical settings.
Strengths: No withdrawal required, can demonstrate generality, and applicable across many clinical scenarios. Limitations: Requires that baselines remain stable and independent of each other until the intervention is introduced. If baselines covary, the demonstration of experimental control is weakened.
Alternating Treatments Design
The alternating treatments design (ATD) compares two or more interventions by rapidly alternating them. Each condition is associated with a distinct discriminative stimulus (such as a therapist, time of day, or setting) to help the participant distinguish between conditions.
When to use it: Use the ATD when you want to compare the relative effectiveness of two or more interventions for the same behavior. It is efficient because it does not require lengthy baseline or reversal phases.
Counterbalancing: To control for sequence effects, the order of conditions should be randomized or counterbalanced across sessions.
Strengths: Rapid comparison of treatments, no extended baseline needed, and each phase serves as a control for the other. Limitations: Carryover effects can occur, and the design requires that conditions be clearly discriminable. It also assumes the behavior is sensitive to the different conditions within a short timeframe.
Changing Criterion Design
The changing criterion design evaluates the effect of an intervention by gradually adjusting the criterion for success. Each time the participant meets the criterion, the bar is raised (or lowered) to a new level.
When to use it: This design is best suited for shaping, where behavior change occurs incrementally. It works well for behaviors that can be measured on a continuum, such as exercise duration, number of math problems completed, or cigarettes smoked per day.
Bidirectional changes: To strengthen the demonstration of experimental control, some phases may involve temporarily lowering the criterion before raising it again.
Strengths: No withdrawal of treatment required, intuitive to implement, and closely mirrors the natural process of shaping. Limitations: Only appropriate for behaviors that change gradually, and the demonstration of control depends on behavior closely matching each new criterion.
Multielement Design
The multielement design (sometimes called a multi-element or alternating conditions design) involves rapidly alternating between two or more conditions to compare responding. It is commonly used in functional analysis to compare behavior across test and control conditions.
Distinguishing from ATD: While similar, the multielement design typically compares conditions (such as attention, escape, alone, and play conditions in a functional analysis) rather than comparing treatments. The ATD compares interventions aimed at the same outcome.
When to use it: Use the multielement design when you need to identify the function of a behavior or compare responding across different environmental conditions.
Choosing the Right Design on the BCBA Exam
When the exam presents a scenario and asks you to select a research design, consider these key questions: Is the behavior likely to reverse? (If yes, consider a reversal design.) Is withdrawal of treatment ethical? (If not, consider a multiple baseline.) Do you need to compare two treatments? (Consider an ATD.) Is the behavior expected to change gradually? (Consider a changing criterion design.) Are you trying to identify the function of behavior? (Consider a multielement design.)
Understanding the logic and application of each design will prepare you to answer both knowledge-based and applied questions on the BCBA exam confidently.