A single-subject research design is best described as involving:

• A. Repeated measurements on a single subject
• B. Large multi-site trials
• C. Cross-sectional samples
• D. Systematic reviews

Answer: (A)

Single-subject research designs focus on tracking a behavior within one individual over time to see how it changes with an intervention. By taking repeated measurements during baseline and after introducing the treatment (and sometimes across different conditions or phases), you can observe a clear pattern that links the intervention to the behavior change for that person. This approach emphasizes within-person data and experimental control, rather than comparing groups.

The other options describe approaches that involve many participants or different kinds of evidence: large multi-site trials gather data across groups of people; cross-sectional samples provide a single snapshot in time; systematic reviews synthesize findings from multiple studies. So the description that best fits a single-subject design is repeated measurements on a single subject.