Zayvon has conducted a study in which he found that college students with more friends tend to have higher GPAs. He concluded that as a person makes more friends, she has more social support and that helps her do better academically. However, when he collected longitudinal data in a follow-up study and was able to take into account time order, Zayvon found that, actually, as a person increases her GPA, she gains status on campus and attracts more friends. This is a case of