  In the study of dynamical and physical systems, the input parameters are
often uncertain or randomly distributed according to a measure $\varrho$. The
system's response $f$ pushes forward $\varrho$ to a new measure $f\circ
\varrho$ which we would like to study. However, we might not have access to $f$
but only to its approximation $g$. We thus arrive at a fundamental question --
if $f$ and $g$ are close in $L^q$, does $g\circ \varrho$ approximate $f\circ
\varrho$ well, and in what sense? Previously, we demonstrated that the answer
to this question might be negative in terms of the $L^p$ distance between
probability density functions (PDF). Here we show that the Wasserstein metric
is the proper framework for this question. For any $p\geq 1$, we bound the
Wasserstein distance $W_p (f\circ \varrho , g\circ \varrho) $ from above by
$\|f-g\|_{q}$. Furthermore, we provide lower bounds for the cases of $p=1,2$.
Finally, we apply our theory to the analysis of common numerical methods in the
field of computational uncertainty quantification.
