First Author:   Valerie Mignon University of Paris X - Nanterre - THEMA Valerie.Mignon@u-paris10.fr   200 avenue de la Republique Nanterre, 92001, France   33 1 40 97 58 60(Phone) 33 1 40 97 58 60(Fax) Second Author: Sandrine Lardic University of Paris X - Nanterre - MODEM Sandrine.Lardic@u-paris10.fr   200 avenue de la République Nanterre, 92001, France