)٥( Retherford, Robert D. and Minja Kim Choe, Statistical Models for Causal Analysis (John Wiley and Sons, Inc. 1993), pp. 20-21.
5 Retherford, Robert D. y Minja Kim Choe, Statistical Models for Causal Analysis (John Wiley and Sons, Inc., 1993), págs. 20 y 21.