Concentration ratio

In economics, the concentration ratio of an industry is used as an indicator of the relative size of firms in relation to the industry as a whole. This may also assist in determining the market form of the industry. One commonly used concentration ratio is the four-firm concentration ratio, which consists of the market share, as a percentage, of the four largest firms in the industry. In general, the N-firm concentration ratio is the percentage of market output generated by the N largest firms in the industry.

The concentration ratio has a fair amount of correlation to the Herfindahl index, another indicator of firm size.

Some examples of the four-firm concentration ratio include:

Traditional agriculture: Less than 5 percent
Sheet metal: 9 percent
Asphalt paving: 15 percent
Typesetting: 16 percent
Publishing: 23 percent
Soap and detergents: 63 percent
Men's slacks: 75 percent
Aircraft: 79 percent
Greeting cards: 84 percent
Cigarettes: 93 percent

Market forms can often be classified by their concentration ratio. Listed, in ascending firm size, they are:

Perfect competition, with a very low concentration ratio,
Monopolistic competition, below 40 percent for the four-firm measurement,
Oligopoly, above 40 percent for the four-firm measurement, and
Monopoly, with a near-100 percent four-firm measurement.