In an A-B-A-B single-subject design, what does A represent and what does B represent?

• A. A represents baseline measurements and B represents post-treatment measurements.
• B. A represents post-treatment measurements and B represents baseline measurements.
• C. A represents multiple baseline measurements and B represents multiple post-treatment measurements.
• D. A and B are random phases with no defined meaning.

Answer: (C)

In this type of single-subject design, the A and B labels describe phases of the experiment. The A phase represents baseline data collection—measuring the target behavior without the intervention to establish how the behavior looks before any treatment. The B phase represents the introduction of the treatment, during which you observe how the behavior changes when the intervention is applied.

In an A-B-A-B sequence, this pattern repeats: baseline, apply treatment, withdraw treatment (return to baseline), and then reapply treatment. The idea is that changes in the behavior align with when the treatment is present, and revert when it’s removed, which helps show that the treatment is affecting the behavior rather than other factors.

The concept you’re aiming for is that A is baseline measurements and B is the treatment phase. Terms like “multiple baseline” refer to a different design structure across multiple targets or settings, not the labeling of these phases within a single sequence, and “post-treatment” would not capture the treatment-on vs. treatment-off dynamics that define A and B.