Amira Majoul Higher Institute of Management of Tunis-University of Tunis - Tunisia   majoul_amira@hotmail.com tunis, tunisia 2000 tunisia   majoul_amira@hotmail.com   www.isg.rnu.tn           JEL Areas: C1 : Econometric and Statistical Methods: General E3 : Prices, Business Fluctuations, and Cycles: General (includes Measurement and Data)