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Herfindahl-Hirschman Index (HHI)
A measure of market concentration, it depends on the number of firms and their size relative to the market. It is calculated by summing up the squares of market shares of each firm. For example, a market where the HHI comes to more than 1800 will be considered a concentrated market. Mergers or acquisitions that change the HHI by more than 100 points in a concentrated market may raise antitrust concerns within the Department of Justice.
An index of market concentration the U.S. Department of Justice uses to determine whether a monopoly is forming. The scale goes from zero to 10,000, with 10,000 indicating that a single company controls 100% of the market share in a given industry. An HHI of less than 1000 indicates a market with little concentration, which the Justice Department prefers. Any merger or acquisition leading to an increase of more than 100 when the HHI was previously greater than 1,800 may lead to antitrust action against the company involved.