Cardiologists at Athens Medical School in Greece wanted to test if chocolate affects blood vessel function. The researchers recruited 17 healthy young volunteers, who were each given a 3.5-ounce bar of dark chocolate, either bittersweet or fake chocolate. On another day, the volunteers received the other treatment. The order in which subjects received the bittersweet and fake chocolate was determined at random. The subjects had no chocolate outside the study, and investigators didn’t know if a subject had eaten the real or the fake chocolate. An ultrasound was taken of each volunteer’s upper arm to observe the functioning of the cells in the walls of the main artery. The researchers found that blood vessel function was improved when the subjects ate bittersweet chocolate, and that there were no such changes when they ate the placebo (fake chocolate).

a. What type of design did the researchers use in their study?

b. Explain why the researchers chose this design instead of a completely randomized design.

c. Why is it important to randomly assign the order of the treatments for the subjects?

d. Explain how and why researchers controlled for other variables in this experiment.