Geographic market definition has become increasingly important in many industries. This is due not only to its managerial implications, but also because of its importance in antitrust policy. The effect of different geographic definitions on market share and the systematic differences among them is the major focus of this research. The analysis is conducted at the store-level on five geographic market definitions, in twelve metropolitan areas, for two large retail grocery chains. Market share is measured on two dimensions. One measure is sales per week for each store. The second measure is square feet of selling area for each store. The sample consists of 9,420 supermarkets type five from the 1997 TradeDimensions database A pilot study and a complete research were conducted. Significant differences among the definitions were found but no evidence of systematic differences was proved in the pilot study and the complete study
