Post-acquisition performance in the short and long run. Evidence from the Copenhagen Stock Exchange 1993-1997

The paper investigates the short-run price adjustment around acquisition announcements and the long-run upward bias of cross-sectional average buy-and-hold returns. The geometric Brownian motion model is applied to decompose the cross-sectional average long-run returns into transformed mean and volatility components. The decomposition improves the interpretation of security performance. The methodology is demonstrated on the security performance of bidding firms listed on the Copenhagen Stock Exchange. The most surprising finding is that the long-run abnormal return after three years is not significantly different from zero. This implies that the bidding firms do not under-perform relative to the market. This result stands in contrast to findings in other studies and it may reflect that earlier studies do not adjust correctly for the volatility component. These current findings indicate that the market efficiency hypothesis is intact in the long run. It is only in the very short run, a few days around acquisition announcements, that the market makes a significant adjustment to uphold the efficiency hypothesis.