Concentration Ratio
In economics, a ratio that indicates the relative size of firms in relation to their industry as a whole.
Investopedia Commentary
The concentration ratio indicates whether an industry is comprised of a few large firms or many small firms. The four-firm concentration ratio, which consists of the market share (expressed as a percentage) of the four largest firms in an industry, is a commonly used concentration ratio. The Herfindahl index, another indicator of firm size, has a fair amount of correlation to the concentration ratio.
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See also: Dog Eat Dog, Economics, Herfindahl-Hirschman Index - HHI, Microeconomics, Monopolistic Competition, Monopoly, Oligopoly, Perfect Competition