For the present purpose of discussing financial stability analysis, however, we need not dwell on these differences. 34  The HHI is calculated by squaring the market share of each firm in a particular market and then adding those squared market shares.
The antitrust agencies have specified the relative level of scrutiny that can be expected from particular combinations of changes in, and post-merger HHIs, although they are careful to point out that HHI is not a “‘rigid screen.”‘ U.S. Department of Justice, Horizontal Merger Guidelines §5.3 (Aug. 19, 2009).