Predicting Merger Outcomes: The Accuracy of Stock Market Event Studies, Market Structure Characteristics, and Agency Decisions

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Merger analysis is an exercise in prediction. This paper analyzes the accuracy of two leading methods of predicting merger outcomes—stock market event studies and an approach using market structure criteria—as well as the accuracy of antitrust agencies’ decisions about challenging mergers. The basis for these evaluations is a database of price effects of 41 mergers compiled from published retrospective studies of mergers. This paper finds that event studies systematically underpredict the incidence of anticompetitive outcomes, while market structure criteria overpredict competitive problems. Agency decisions correct much of that overprediction, however, which suggests that structural criteria may serve as an appropriate first screen.

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