Questions: 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 55% reduction in their WASO, whereas RT-treated and placebo-treated patients, respectively, achieved only 17% and 13% 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. Answer parts (a) through (f) below. (a) What type of experimental design is this? A. Completely randomized design B. Matched-pairs design C. Cohort study D. Randomized block design

The answer is A: Completely randomized design.

Explanation for each option:

A. Completely randomized design: This is the correct answer because the researchers randomly assigned the 75 patients into three different treatment groups (CBT, RT, and placebo) without any specific blocking or matching criteria. Each patient had an equal chance of being assigned to any of the three groups, which is characteristic of a completely randomized design.

B. Matched-pairs design: This option is incorrect because a matched-pairs design involves pairing subjects based on certain characteristics and then randomly assigning each member of the pair to different treatments. In this study, there is no indication that patients were paired based on any criteria before being assigned to the treatment groups.

C. Cohort study: This option is incorrect because a cohort study typically involves following a group of people over time to observe outcomes without manipulating the study environment. In this case, the researchers actively assigned treatments to the participants, which is an experimental design rather than an observational cohort study.

D. Randomized block design: This option is incorrect because a randomized block design involves dividing subjects into blocks based on certain characteristics and then randomly assigning treatments within each block. There is no mention of blocking based on characteristics in this study; the assignment was purely random across the entire sample.