Under antitrust policy, any merger that increases this number by more than 100 or causes its value to exceed 1000 is considered anticompetitive, as it is below 1000 for an open market.  
Under antitrust policy, any merger that increases this number by more than 100 or causes its value to exceed 1000 is considered anticompetitive, as it is below 1000 for an open market.  It is above 1800 for a concentrated market and equal to 10,000 for a pure monopoly.  It is calculated by taking the sum of the squares of each firm's market share.  FTP, name this index which measures the degree to which an industry is concentrated in a particular number of firms.
Under antitrust policy, any merger that increases this number by more than 100 or causes its value to exceed 1000 is considered anticompetitive, as it is below 1000 for an open market.  
Herfindahl-Hirschman index