First Author:   Valerie Mignon University of Paris 10 - THEMA Valerie.Mignon@u-paris10.fr   200 avenue de la Republique, THEMA - CNRS Nanterre Cedex, 92001, France   33 1 40 97 58 60(Phone) 33 1 40 97 58 60(Fax) Second Author: Sandrine Lardic University of Paris 10 - MODEM Sandrine.Lardic@u-paris10.fr