Generating evidence to guide merger enforcement

The challenge of effective merger enforcement is tremendous. U.S. Antitrust agencies must, by statute, quickly forecast the competitive effects of mergers that occur in virtually every sector of the economy to determine if mergers can proceed. Surprisingly, given the complexity of the regulators' task, there is remarkably little empirical evidence on the effects of mergers to guide regulators. This paper describes the need for retrospective analysis of past mergers in building an empirical basis for antitrust enforcement, and provides guidance on the key measurement issues researchers confront in estimating the price effects of mergers. We also describe how evidence from merger retrospectives can be used to evaluate the economic models that predict the competitive effects of mergers.