For over two decades, the Department of Justice and the Federal Trade Commission have used Guidelines to categorize markets for purposes of merger analysis. Based on extensive theoretical and empirical evidence, the Merger Guidelines define a market with the equivalent of 10 or more equal-sized competitors as unconcentrated. The Guidelines define markets with the equivalent of between 6 and 10 equal-sized competitors as moderately concentrated. They define a market with the equivalent of fewer than 6 equal-sized competitors as highly concentrated. In concentrated markets the threat of the abuse of market power is substantial.
