challenge it. If a merger would result in an HHI between 1,000 and 1,800, then the FTC would scrutinize the

plan and make a case-by-case decision. However, in the last several decades, the antitrust enforcement

authorities have moved away from relying as heavily on measures of concentration ratios and HHIs to

determine whether they will allow a merger, and instead they carry out more case-by-case analysis on the

extent of competition in different industries.

New Directions for Antitrust

Both the four-firm concentration ratio and the Herfindahl-Hirschman index share some weaknesses. First,

they begin from the assumption that the “market” under discussion is well-defined, and the only question is

measuring how sales are divided in that market. Second, they are based on an implicit assumption that

competitive conditions across industries are similar enough that a broad measure of concentration in the
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