In single-subject research, which design alternates periods with and without intervention to demonstrate causal effects?

• A. ABAB design
• B. Randomized block design
• C. Crossover with no washout
• D. Factorial design

Answer: (A)

The main idea is to show a causal link by watching how a single subject’s behavior changes when the intervention is present versus when it is not, and to repeat those changes to confirm the effect. The design that does this by cycling between baseline (no intervention) and intervention, then removing and reinstating the intervention to see if the behavior follows, is the ABAB design. By reintroducing the intervention after withdrawal and observing a renewed change, you gain stronger evidence that the intervention is driving the behavior rather than other factors.

Other designs don’t provide that same within-subject demonstration. A randomized block design compares groups rather than switching conditions within a single subject. A crossover design without a washout can have carryover effects that muddy whether the treatment itself caused changes. A factorial design looks at multiple factors and their interactions, typically across groups, rather than alternating a single subject through repeated on/off phases.