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If the market shares for replacing automobile windshields are:

The Auto Glass Doctor Company
Your Car Shield Company
Seven firms that each have 6% of the market
Eight firms that each have 3% of the market
Then the four-firm concentration ratio is 16 + 10 + 8 + 6 = 40.

10% of the market
8% of the market
42% of the market, combined 24% of the market, combined

Table 11.1 Calculating Concentration Ratios from Market Shares

The concentration ratio approach can help to clarify some of the fuzziness over deciding when a merger might affect competition. For instance, if two of the smallest firms in the hypothetical market for repairing automobile windshields merged, the four-firm concentration ratio would not change—which implies that there is not much worry that the degree of competition in the market has notably diminished. However, if the top two firms merged, then the four-firm concentration ratio would become 46 (that is, 26 + 8 + 6 + 6). While this concentration ratio is modestly higher, the four-firm concentration ratio would still be less than half, so such a proposed merger might barely raise an eyebrow among antitrust regulators.

The Herfindahl-Hirshman Index

A four-firm concentration ratio is a simple tool, which may reveal only part of the story. For example, consider two industries that both have a four-firm concentration ratio of 80. However, in one industry five firms each control 20% of the market, while in the other industry, the top firm holds 77% of the market and all the other firms have 1% each. Although the four-firm concentration ratios are identical, it would be reasonable to worry more about the extent of competition in the second case—where the largest firm is nearly a monopoly—than in the first.

Another approach to measuring industry concentration that can distinguish between these two cases is called the Herfindahl-Hirschman Index (HHI). We calculate HHI by summing the squares of the market share of each firm in the industry, as the following Work It Out shows.