  Isospin transport ratio is a powerful method to estimate the neutron-proton
(n-p) equilibration in heavy-ion collisions, and extensively used to obtain
information on the asy-stiffness of the nuclear Equation of State. In fact such
a ratio is expected to bypass any perturbations introducing a linear
transformation of the chosen observable. In particular, it is supposed to
overcome contributions due to emission, either of dynamical or statistical
nature, from the primary fragments formed during the collisions. In this paper
we explore the validity of this assumption, looking at the quasi-projectile n-p
ratio ($N/Z$) in peripheral and semi-peripheral events for Ca+Ca reactions at
35\amev{}, simulated via the Antisymmetrized Molecular Dynamics transport
model, coupled to different statistical decay codes. The statistical
de-excitation of the primary fragments introduces a linear transformation at
relatively high excitation energies (above 2\amev{}) when the residue
approaches the Evaporation Attractor Line, while some effect is produced at
lower excitation energies due to the occurrence of some non-linearities. As for
fast emissions after the end of the projectile-target interaction it is shown
that they introduce a non-linear transformation too.
