Chequeo de la estandarización usada en Gretl
sd(A_z) = 1,000000   sd(B_z) = 1,000000   sd(C_z) = 1,000000   sd(D_z) = 1,000000
mean(A_z) = 0,000000   mean(B_z) = -0,000000   mean(C_z) = 0,000000   mean(D_z) = 0,000000