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A '''concentration ratio''' is a measure of the extent to which a market or industry is dominated by a small number of firms. It is commonly expressed as the combined market share of the largest firms, such as the four-firm concentration ratio, which is the sum of the market shares of the four largest firms, or the eight-firm ratio. Concentration ratios are used in competition policy, in the analysis of market structure, and in research on the relationship between market structure and performance.

== Measures ==

The most common concentration ratios are the n-firm ratios, which sum the market shares of the n largest firms. A four-firm ratio of eighty per cent indicates that the four largest firms account for eighty per cent of the market, suggesting a concentrated market, while a ratio of twenty per cent indicates a competitive one. The choice of n affects the measure, and different values provide different perspectives on the structure of the market.

Other measures include the Herfindahl-Hirschman index, which is the sum of the squared market shares of all firms and which gives more weight to the largest firms, and the Lorenz curve and Gini coefficient, which describe the distribution of shares across firms. Each measure has different properties, and the choice depends on the purpose of the analysis and on the data available.

== Uses in competition policy ==

Concentration ratios are used in competition policy to assess the structure of markets and to evaluate the effects of mergers. A merger that would substantially increase concentration may raise concerns about the exercise of market power, and the analysis of concentration is part of the assessment of whether a merger should be permitted. The measures provide a simple summary of market structure, and they are used alongside other evidence on competition.

The use of concentration measures has limitations. They describe the structure of the market but do not directly measure competition, since competition depends on the behaviour of firms, on the ease of entry, and on the closeness of substitutes. A concentrated market may be competitive if entry is easy and if firms compete vigorously, while a market with many firms may be uncompetitive if they coordinate. Concentration measures are therefore a starting point for analysis rather than a conclusion.

== Determinants and interpretation ==

The concentration of a market depends on several factors. The minimum efficient scale, relative to the size of the market, determines how many firms can operate at low cost, and where it is large, the market tends to be concentrated. Barriers to entry, including economies of scale, control of essential inputs, and regulatory requirements, affect the number of firms. The nature of the product, including whether it is homogeneous or differentiated, also influences the structure.

The interpretation of concentration measures requires attention to the definition of the market, since concentration depends on how the market is defined in terms of products and geography. A narrowly defined market may appear concentrated, while a broadly defined one may not. The analysis must therefore consider the boundaries of the market and the substitutability of products, and it must be cautious in drawing conclusions about competition from concentration alone.

== Applications ==

Concentration ratios are used in research on the relationship between market structure and performance, on the effects of mergers, and on the determinants of profitability. The evidence suggests that concentration is associated with higher prices and profits in some industries, though the relationship is not universal and depends on the contestability of the market and on the behaviour of firms.

The measures are also used in policy to screen mergers for further review, to identify markets that may require attention, and to monitor changes in structure over time. The choice of thresholds and the design of the remedies draw on the analysis of concentration and of its implications, and the measures provide a common language for discussing the structure of markets, even as the analysis of competition relies on a broader set of evidence.

== Types and Construction ==
Concentration ratios can be constructed for different measures, including sales, output, employment, value added, or capacity. The choice of measure affects the result, especially where firms differ in vertical integration or product mix. Ratios based on domestic production differ from those based on domestic sales, which include imports, and the difference matters for evaluating import competition.

Concentration can also be measured at national, regional, or global levels. A market defined narrowly may appear highly concentrated, while a broader definition yields lower concentration. The geographic and product boundaries of the market are therefore critical and often contested in competition cases.

== Relation to Other Concentration Measures ==
The four-firm and eight-firm concentration ratios are simple and intuitive but discard information about the distribution of shares among the leading firms and about the number of smaller firms. The Herfindahl-Hirschman index, which sums the squared market shares of all firms, captures more detail and is the standard measure used by competition authorities in many jurisdictions. The entropy index and the comprehensive concentration index are alternatives with different properties.

Each measure has different sensitivity to the number of firms and to inequality among them. Comparisons across markets or over time should therefore use consistent measures and market definitions.

== Limitations ==
Concentration ratios convey little about the intensity of competition by themselves. A market with a high concentration ratio may nonetheless be competitive if entry is easy or if the leading firms compete vigorously, while a fragmented market may feature coordinated behaviour or high barriers to entry. Concentration is a structural indicator, not a direct measure of conduct or performance.

Data limitations also arise. In many markets the shares of privately held firms are not observed, and reported shares may be based on different definitions. Cross-border ownership and multi-market operations complicate the allocation of shares to a single market.

== Policy Relevance ==
Despite these limitations, concentration ratios are widely used in merger review, market studies, and regulatory oversight. Regulators use them as screening tools to identify markets warranting closer examination, combined with analysis of entry conditions, buyer power, and the scope for coordination. The measure also features in debates about the effects of consolidation on prices, innovation, and the distribution of economic power, and about the appropriate role of competition policy in digital and platform markets where concentration has risen in recent years.

== See also ==

* [[Market Structure]]
* [[Market Power]]
* [[Herfindahl-Hirschman Index]]
* [[Competition Policy]]
* [[Barriers to Entry]]

== References ==

<ref name="r1">Industrial organisation textbooks on market structure.</ref>
<ref name="r2">OECD and competition authority guidance on concentration.</ref>
<ref name="r3">Journal literature on concentration and performance.</ref>
<ref name="r4">Regulatory studies of merger review.</ref>

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[[Category:Industrial organization]]
[[Category:Market structure]]
[[Category:Microeconomics]]