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An oligopoly is a market form in which a market or industry is dominated by a small number of sellers (oligopolists). The word is derived, by analogy with "monopoly", from the Greek ὀλίγοι (oligoi) "few" + πωλειν (polein) "to sell". Because there are few sellers, each oligopolist is likely to be aware of the actions of the others. The decisions of one firm influence, and are influenced by, the decisions of other firms. Strategic planning by oligopolists needs to take into account the likely responses of the other market participants.



Oligopoly is a common market form. As a quantitative description of oligopoly, the four-firm concentration ratio is often utilized. This measure expresses the market share of the four largest firms in an industry as a percentage. For example, as of fourth quarter 2008, Verizon, AT&T, Sprint Nextel, and T-Mobile together control 89% of the US cellular phone market.

Oligopolistic competition can give rise to a wide range of different outcomes. In some situations, the firms may employ restrictive trade practices (collusion, market sharing etc.) to raise prices and restrict production in much the same way as a monopoly. Where there is a formal agreement for such collusion, this is known as a cartel. A primary example of such a cartel is OPEC which has a profound influence on the international price of oil.

Firms often collude in an attempt to stabilize unstable markets, so as to reduce the risks inherent in these markets for investment and product development.[citation needed] There are legal restrictions on such collusion in most countries. There does not have to be a formal agreement for collusion to take place (although for the act to be illegal there must be actual communication between companies)–for example, in some industries there may be an acknowledged market leader which informally sets prices to which other producers respond, known as price leadership.

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