SELLER-CONCENTRATION plays a major role in most theories of market-behavior and market-performance. As a result, most empirical studies of industrial organization include a variable representing the degree of seller-concentra- tion in relevant markets. Unfortunately, seller-concentration cannot be measured easily: Market-boundaries are difficult to identify, and market- shares of particular firms are difficult to calculate. Therefore, most concen- tration variables are mere proxies for true seller-concentration.