Using Single Subject Experimental Designs

single subject experimental designs applied behavior analysis

What are the Characteristics of Single Subject Experimental Designs?

Single-subject designs are the staple of applied behavior analysis research. Those preparing for the BCBA exam or the BCaBA exam must know single subject terms and definitions. When choosing a single-subject experimental design, ABA researchers are looking for certain characteristics that fit their study. First, individuals serve as their own control in single subject research. In other words, the results of each condition are compared to the participant’s own data. If 3 people participate in the study, each will act as their own control. Second, researchers are trying to predict, verify, and replicate the outcomes of their intervention. Prediction, replication, and verification are essential to single-subject design research and help prove experimental control.

Prediction: the hypothesis related to what the outcome will be when measured
Verification: showing that baseline data would remain consistent if the independent variable was not manipulated
Replication: repeating the independent variable manipulation to show similar results across multiple phases

Some experimental designs like withdrawal designs are better suited for demonstrating experimental control than others, but each design has its place. We will now look at the different types of single subject experimental designs and the core features of each.

Reversal Design/Withdrawal Design/A-B-A

Arguably the simplest single subject design, the reversal/withdrawal design is excellent at identifying experimental control. First, baseline data is recorded. Then, an intervention is introduced and the effects are recorded. Finally, the intervention is withdrawn and the experiment returns to baseline. The researcher or researchers then visually analyze the changes from baseline to intervention and determine whether or not experimental control was established. Prediction, verification, and replication are also clearly demonstrated in the withdrawal design. Below is a simple example of this A-B-A design.

reversal design withdrawal design
Reversal/Withdrawal Design

Advantages: Demonstrate experimental control
Disadvantages: Ethical concerns, some behaviors cannot be reversed, not great for high-risk or dangerous behaviors

Multiple Baseline Design/Multiple Probe Design

Multiple baseline designs are used when researchers need to measure across participants, behaviors, or settings. For instance, if you wanted to examine the effects of an independent variable in a classroom, in a home setting, and in a clinical setting, you might use a multiple baseline across settings design. Multiple baseline designs typically involve 3-5 subjects, settings, or behaviors. An intervention is introduced into each segment one at a time while baseline continues in the other conditions. Below is a rough example of what a multiple baseline design typically looks like:

multiple baseline design single subject design
Multiple baseline design

Multiple probe designs are identical to multiple baseline designs except baseline is not continuous. Instead, data is taken only sporadically during the baseline condition. You may use this if time and resources are limited, or you do not anticipate baseline changing.

Advantages: No withdrawal needed, examine multiple dependent variables at a time
Disadvantages: Sometimes difficult to demonstrate experimental control

Alternating Treatment Design

The alternating treatment design involves rapid/semirandom alternating conditions taking place all in the same phase. There are equal opportunities for conditions to be present during measurement. Conditions are alternated rapidly and randomly to test multiple conditions at once.

alternating treatment design applied behavior analysis
Alternating treatment design

Advantages: No withdrawal, multiple independent variables can be tried rapidly
Disadvantages: The multiple treatment effect can impact measurement

Changing Criterion Design

The changing criterion design is great for reducing or increasing behaviors. The behavior should already be in the subject’s repertoire when using changing criterion designs. Reducing smoking or increasing exercise are two common examples of the changing criterion design. With the changing criterion design, treatment is delivered in a series of ascending or descending phases. The criterion that the subject is expected to meet is changed for each phase. You can reverse a phase of a changing criterion design in an attempt to demonstrate experimental control.

changing criterion design aba
Changing criterion design

Summary of Single Subject Experimental Designs

Single subject designs are popular in both social sciences and in applied behavior analysis. As always, your research question and purpose should dictate your design choice. You will need to know experimental design and the details behind single subject design for the BCBA exam and the BCaBA exam. For BCBA exam study materials check out our BCBA exam prep. For a full breakdown of the BCBA fifth edition task list, check out our YouTube: