Social Psychology 9th Edition by David Myers

Exercise 87

You conduct a study that examines the role of exercise on depression alleviation. You assign the first 50 people who are motivated to sign up to the experimental group, and the second group of 50 people, who sign up much later, to the control group. After one month, you find that the experimental group (who exercised three times a week on average) is significantly less depressed than the control group (who exercised one time a week on average). Although you may be tempted to conclude that exercise helps stave off depression, you cannot because of a lack of ____ in your study.
A) random sampling
B) random assignment
C) random surveying
D) objectivity