Researchers wanted to test the effectiveness of a new cognitive behavioral therapy (CBT) compared with both an older behavioral treatment and a placebo therapy for treating insomnia. They identified 75 adults with insomnia. Patients were randomly assigned to one of three treatment groups. Twenty-five patients were randomly assigned to receive CBT (sleep education, stimulus control, and time-in-bed restrictions), another 25 received muscle relaxation training (RT), and the final 25 received a placebo treatment. Treatment lasted 6 weeks, with follow-up conducted at 6 months. To measure the effectiveness of the treatment, researchers used wake time after sleep onset (WASO). CBT produced larger improvements than did RT or placebo treatment. For example, the CBT-treated patients achieved an average 54% reduction in their WASO, whereas RT-treated and placebo-treated patients, respectively, achieved only 16% and 12% reductions in this measure. Results suggest that CBT treatment leads to significant sleep improvements within 6 weeks and these improvements appear to endure through 6 months of follow-up.

(a) What type of experimental design is this?
(b) What is the population being studied?
(c) What is the response variable in this study?
(d) What are the treatments?
(e) Identify the experimental units.
(f) Draw a diagram similar to Figure 7 or 8 to illustrate the design.

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Figure 7 Random assignment of plants to treatments Group 1 receives 20 plants Group 2 receives 20 plants Group 3 receives 20 plants Treatment A: No fertilizer Treatment B: 2 teaspoons Treatment C: 4 teaspoons Compare yield