First Author:   Mahmoud Sami Nabi LEGI – Tunisia Polytechnic School and University of Sousse - IHEC nabi@univ-paris1.fr   32 Rue Hassen Kenani, Menzah 9A Tunis, 1013, Tunisia   +216 21 91 81 16(Phone) +216 21 91 81 16(Fax)