The tendency of a market to be dominated by a few big firms. A high degree of concentration may be evidence of antitrust problems, if it reflects a lack of competition. Traditionally, economists examined whether there was too much concentration using the Herfindahl-Hirschman index, which is determined by adding the squares of the market shares of all firms involved. A low Herfindahl indicated many competitors and thus great difficulty in exercising market power; a high Herfindahl, however, suggested a concentrated market in which price rises are easier to sustain. More recently, antitrust authorities have placed less emphasis on concentration. One reason is that it is hard to define the market in which concentration should be measured. Instead, antitrust authorities have turned their attention to finding examples of firms earning excessive profits or holding back innovation, although this too raises tricky conceptual and practical questions.