You conduct a study on a group of individuals to examine the role of exercise in alleviating depression. 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 people in the experimental group (who exercised three times a week on average) are significantly less depressed than the people in the control group (who exercised once a week on average). Although you may be tempted to conclude that exercise helps stave off depression, you cannot because of the lack of in your study.