Final answer:
Aaron Beck's work on the cognitive triad is fundamental in understanding depression and its treatment through cognitive-behavioral therapy, and his experimental approach using randomized controlled trials is key in validating the effectiveness of these therapies.
Step-by-step explanation:
Aaron Beck has argued that the cognitive triad, which includes thoughts about the self, the world, and the future, is a powerful determinant of mental health and levels of depression. The cognitive triad is a crucial concept in cognitive-behavioral therapy (CBT), which Beck developed as a treatment for depression. When Beck screens participants and randomly assigns them to treatment or control groups, he is most likely conducting a randomized controlled trial (RCT), a type of scientific experiment that is considered the gold standard for testing the efficacy of medical interventions including psychotherapy techniques like CBT.