A gym that specializes in weight loss offers its members an optional dietary program for an extra fee. To study the effectiveness of the dietary program, a manager at the gym takes a random sample of 505050 members who participate in the dietary program and 505050 members who don't. The manager finds that those who participated in the dietary program, on average, lost significantly more weight than those who didn't participate in the program in the past 333 months. The manager concludes that the dietary program caused the increased weight loss for the gym's members during that time period.
What is a confounding variable that makes their conclusion inappropriate?