First Author:   Thouraya Boujelbene Economics thourayaboujelbene@yahoo.fr   Route de téniour Km 7 Chihia, 3041, Tunisia   +21698974240(Phone) +21698974240(Fax) Second Author: Younes Boujelbène Economics Younes.Boujelbène@fsegs.rnu.tn   Route de Teniour km 5.5 Chihia, 3041, Tunisia   +21622684420(Phone) +21674279139(Fax)