First Author:   Maiga Nouhoun Oumarou University of Thomas Sankara nouhounmaiga87@yahoo.com   Ouagadougou, Burkina Faso   00226 70027541(Phone) 00226 70027541(Fax) Second Author: Sirpe Gnanderman University of Thomas Sankara gsirpe@yahoo.fr   Ouagadougou, Burkina Faso   00226 70263468(Phone)