Students who participate in extracurricular activities at university tend to achieve higher grades than those who do not. A recent study tracked 2,000 students over three years and found that those involved in sports teams, debating societies, or music groups averaged 8% higher marks than their peers who focused solely on academic work. The study controlled for prior academic achievement by comparing students with similar A-level results. Universities should therefore encourage all students to join extracurricular activities, as this will improve their academic performance. Some institutions have already begun making participation in at least one society mandatory for first-year students, and early results suggest this policy is raising overall grade averages. Which one of the following best identifies the flaw in the above argument?