First Author:   Issa Dianda Thomas Sankara University, Burkina Faso isdianda@yahoo.fr   Ouagadougou, Burkina Faso   +226 70 59 50 53(Phone) +226 70 59 50 53(Fax) Second Author: Idrissa Ouedraogo Pan African University and the University Yaoundé II,Cameroon pridrissoued87@yahoo.fr   Yaoundé, Cameroon