Is doing tai chi, a traditional Chinese martial art, good for the immune system? The authors of a paper that appeared in the Journal of the American Medical Association (2007, pages 511– 517) describe an experiment in which 112 healthy older adults were assigned at random to one of two groups. One group was the tai chi group. People in this group did tai chi for 40 minutes, three times a week, for 16 weeks. At the end of the 16 weeks, they received a vaccine designed to protect against a certain virus. Nine weeks later, a measurement of immunity to the virus was made. People in the second group served as a control group. They did not do tai chi during the first 16 weeks but received the virus vaccine at the same time as the tai chi group, and their immunity was also measured nine weeks later.
a. Briefly describe why it was important for the researchers to assign participants to one of the two groups rather than letting the participants choose which group they wanted to be in.
b. Explain why it was important to include a control group in this experiment.