Source: https://www.researchgate.net/profile/Helene-Erkel-Rousse/publication/289671176_Introduction_a_l'econometrie_du_modele_lineaire_-_chapitre_0_la_demarche_econometrique_eclairee_par_l'histoire/links/56910d4308aed0aed8146bb1/Introduction-a-leconometrie-du-modele-lineaire-chapitre-0-la-demarche-econometrique-eclairee-par-lhistoire.pdf