Which statement about A-B-A-B single-case design is true?

• A. It uses random assignment to groups.
• B. It includes multiple subjects.
• C. It lacks withdrawal phases.
• D. It includes alternating phases of baseline and treatment to observe effects within a single subject.

Answer: (D)

ABAB single-case design tests whether a behavior change tracks with the introduction and withdrawal of an intervention within the same individual. It starts with a baseline period to measure the behavior without treatment, then applies the treatment to see a change. After withdrawing the treatment, the behavior should revert toward baseline if the treatment caused the change. Reintroducing the treatment should again produce the change, reinforcing the causal link. This sequence—baseline, treatment, withdrawal, re-treatment—demonstrates that observed effects are due to the intervention rather than other factors. Other options describe group designs, omit withdrawal phases, or rely on multiple subjects, which do not capture the within-subject reversal logic of ABAB.