The concentration of firms in an industry is of interest to economists, business strategists, and government agencies. Here, we discuss two commonly-used methods of measuring industry concentration: the Concentration Ratio and the Herfindahl-Hirschman Index.
Concentration Ratio (CR)
The concentration ratio is the percentage of market share owned by the largest m firms in an industry, where m is a specified number of firms, often 4, but sometimes a larger or smaller number. The concentration ratio often is expressed as CRm, for example, CR4.
The concentration ratio can be expressed as:
CRm = s1 + s2 + s3 + ... ... + sm
where si = market share of the ith firm.
If the CR4 were close to zero, this value would indicate an extremely competitive industry since the four largest firms would not have any significant market share.
In general, if the CR4 measure is less than about 40 (indicating that the four largest firms own less than 40% of the market), then the industry is considered to be very competitive, with a number of other firms competing, but none owning a very large chunk of the market. On the other extreme, if the CR1 measure is more than about 90, that one firm that controls more than 90% of the market is effectively a monopoly.
While useful, the concentration ratio presents an incomplete picture of the concentration of firms in an industry because by definition it does not use the market shares of all the firms in the industry. It also does not provide information about the distribution of firm size. For example, if there were a significant change in the market shares among the firms included in the ratio, the value of the concentration ratio would not change.
Herfindahl-Hirschman Index (HHI)
The Herfindahl-Hirschman Index provides a more complete picture of industry concentration than does the concentration ratio. The HHI uses the market shares of all the firms in the industry, and these market shares are squared in the calculation to place more weight on the larger firms. If there are n firms in the industry, the HHI can be expressed as:
HHI = s12 + s22 + s32 + ... ... + sn2
where si is the market share of the ith firm.
For example, for a market consisting of four firms with shares of thirty, thirty, twenty and twenty percent, the HHI is 2600 (302 + 302 + 202 + 202 = 2600).
The HHI takes into account the relative size and distribution of the firms in a market and approaches zero when a market consists of a large number of firms of relatively equal size. The HHI increases both as the number of firms in the market decreases and as the disparity in size between those firms increases.
Unlike the concentration ratio, the HHI will change if there is a shift in market share among the larger firms. Markets in which the HHI is between 1000 and 1800 points are considered to be moderately concentrated, and those in which the HHI is in excess of 1800 points are considered to be concentrated. Transactions that increase the HHI by more than 100 points in concentrated markets presumptively raise antitrust concerns.
Other Considerations in Using Industry Concentration Measures
One should be aware that these measures are influenced by the definition of the relevant market. For example, the automotive industry is not the same as the market for sport utility vehicles. One also must consider the geographic scope of the market, for example, national markets versus local markets.
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