A basic contention of industrial organization theory is that firm numbers and the distribution of market shares among firms are important determinants of industry prices and profit. This proposition is supported by numerous empirical studies showing a positive relationship between concentration and prices, concentration and profitability, and between concentration and price cost margins. Most of these studies utilize 4-firm concentration ratios as measures of firm share distribution. As Kwoka [13, 14] has recently noted, however, there is no theoretical basis for supposing that only 4 firms are relevant to industry performance, nor is there any reason to presume that 4 firms are equally important, as is implied in the construction of concentration ratios.