Lamia Soltani University of Manouba, Quantitative Analysis Research Group (QUARG)   Campus Universitaire de la Manouba Manouba, 2010 Tunisia   lamia.soltani@esen.tn   http://www.uma.rnu.tn/           JEL Areas: C : Mathematical and Quantitative Methods Q0 : Agricultural and Natural Resource Economics; Environmental and Ecological Economics: General