  Using analytical arguments and computer simulations we show that the
dependence of the hopping carrier mobility on the electric field
$\mu(F)/\mu(0)$ in a system of random sites is determined by the localization
length $a$, and not by the concentration of sites $N$. This result is in
drastic contrast to what is usually assumed in the literature for a theoretical
description of experimental data and for device modeling, where $N^{-1/3}$ is
considered as the decisive length scale for $\mu(F)$. We show that although the
limiting value $\mu(F \rightarrow 0)$ is determined by the ratio $N^{-1/3}/a$,
the dependence $\mu(F)/\mu(0)$ is sensitive to the magnitude of $a$, and not to
$N^{-1/3}$. Furthermore, our numerical and analytical results prove that the
effective temperature responsible for the combined effect of the electric field
$F$ and the real temperature $T$ on the hopping transport via spatially random
sites can contain the electric field only in the combination $eFa$.
