The article “Study Points to Benefits of Knee Replacement Surgery Over Therapy Alone” (The New York Times, October 21, 2015) describes a study to compare two treatments for people with knee pain. In the study, 50 people with arthritis received knee replacement surgery followed by a program of exercise. Another 50 people with arthritis did not have surgery but received the same program of exercise. After 1 year, 85% of the people who had surgery and 68% of the people who did not have surgery reported pain relief. 

a. Why would it be important to determine if the researchers randomly assigned the people participating in the study to one of the two groups? 

b. Explain why you think that the researchers did not include a control group (a group that did not receive surgery and also did not receive an exercise program) in this study.