A sociologist conducted a study to determine if a correlation exists between grade point average at the time of graduation and income ten years after graduation. The sociologist visited the alumni office of a local university and obtained the names and addresses of those who had graduated ten years earlier. The sociologist then contacted those graduates and asked them to disclose their income and give permission for the university to disclose their graduating GPAs. Two hundred students replied to this inquiry. The sociologist then applied statistical methods to compute the correlation coefficient, which turned out to be + 0.2. How does this study relate to Mill's methods? What does a correlation coefficient of + 0.2 say about the correlation between GPA and income? Is such a study called a prospective study or a retrospective study? How do the two differ?