The Measurement of Operating Performance Following Corporate Acquisitions : A Comparison of Event-Time to Calendar-Time Measures

We examine a sample of 1,175 firms following their acquisition of another firm. Consistent with previous work, we measure the operating performance of these firms in event-time, and find that they experience significantly positive abnormal operating performance subsequent to their acquisition. When we measure their abnormal operating performance in calendar-time, however, we find that our sample firms' abnormal operating performance is insignificant. In short, we find that the standard event-time measures of abnormal operating performance overstate the benefits of corporate acquisitions