Phu Nguyen Van THEMA-CNRS, Université Cergy-Pontoise   33 boulevard du Port 95011 Cergy-Pontoise Cergy-Pontoise, France   Phu.Nguyen-Van@eco.u-cergy.fr             JEL Areas: C1 : Econometric and Statistical Methods: General Q0 : Agricultural and Natural Resource Economics; Environmental and Ecological Economics: General