Event Study Analysis: Correctly Measuring the Dollar Impact of an Event

Abstract

Using the traditional event study approach in the context of securities litigation, the determination of the materiality of a firm disclosure hinges on the statistical significance of the abnormal share price change (i.e., return) on the disclosure day. To estimate per share damages, the abnormal return is then transformed to an abnormal dollar impact. It is often assumed that if the abnormal return on a disclosure day is statistically significant, so is the abnormal dollar effect. We demonstrate - first analytically and then through an empirical example - that need not be the case. We derive the proper t-statistic if one wishes to determine the statistical significance of an abnormal dollar effect. This has obvious implications for liability and damages in securities litigation matters.