Volker Nocke, Michael D. Whinston, 26 August 2020

Concentration measures such as the post-merger Herfindahl-Hirschman index as well as the merger-induced change in the index are usually key determinants in the review of horizontal mergers by competition agencies and courts. This column studies whether the magnitude of the efficiencies required for a merger not to hurt consumers may be related to the change and the level of the Herfindahl-Hirschman index. On the basis of theoretical analysis substantiated by empirical evidence, it finds that while the critical level of efficiencies depends on the change in the index, it is independent of level of the index. Hence current guidelines should be changed so as to emphasise the change more and the level less.

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