AT&T/T-Mobile Merger: Why DOJ Hates It.donb3 writes "Ever wonder why the Department Justice filed suit against the T-Mobile/AT&T Merger? One possible explanation is the Herfindahl-Hirschman Index. The Herfindahl Index measures how mergers and acquisitions change the competition in a market. Here is a brief explanation of how the Department of Justice measures this index: The Herfindahl-Hirschman Index. Several other proposed mergers have conflicted with this index, including the Mentor/Cadence Graphics merger. Once regulators perform the math, proposed mergers often hit the skids. A brief explanation of the Index is here on Wikipedia, and for those of you who are masochistic, a calculator of your proposed merger is here on the Andrew Chin University of North Carolina School of Law web site."
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