Event studies measure the relationship between an event that affects securities and the return of those securities. Some events, such as a regulatory change or an economic shock, affect many securities contemporaneously; other events, such as a change in dividend policy or a stock split, are specific to individual securities. Event studies are often used to test the efficient market hypothesis. For example, abnormal returns that persist after an event occurs or abnormal returns that are associated with an anticipated event contradict the efficient market hypothesis. Aside from tests of market efficiency, event studies are valuable in gauging the magnitude of an event’s impact. A classic event study published in 1969 by Fama, Fisher, Jensen, and Roll examined the impact of stock splits on security prices. The authors found that abnormal returns dissipated rapidly following the news of stock splits, thus lending support to the efficient market hypothesis.