Verification in singlesubject experimental designs particularly reversal designs ABA or ABAB refers to demonstrating that the behavior would not have changed in the absence of the independent variable IV It typically involves returning the experimental conditions to baseline A phase after the intervention B phase has been implemented If the behavior reverts to baseline levels or follows its initial predicted trend when the IV is withdrawn it verifies the prediction made during the initial baseline phase that the behavior would have remained stable or continued on its existing trajectory had no intervention been introduced This strengthens the inference that the observed changes in behavior during the intervention phase were indeed functionally related to the IV rather than to other uncontrolled factors Option 1 describes replication for external validity or generalization Option 2 describes efforts to control for confounding variables which contributes to internal validity but is not the specific definition of verification Option 4 describes replication specifically the reintroduction of the IV to show its effect again