A numerical twist on the observational spin parameter, $\lambda_R$
  A primary goal of integral field spectroscopic (IFS) surveys is to provide a
statistical census of galaxies classified by their internal kinematics. As a
result, the observational spin parameter, $\lambda_R$, has become one of the
most popular methods of quantifying the relative importance of velocity
dispersion and rotation in supporting a galaxy's inner structure. The goal of
this paper is to examine the relationship between the observationally deduced
$\lambda_R$ and one of the most commonly used theoretical spin parameters in
the literature, the Bullock et al. (2001) $\lambda'$. Using a set of $N$-body
realisations of galaxies from which we construct mock IFS observations, we
measure $\lambda_R$ as an observer would, incorporating the effects of beam
smearing and seeing conditions. Assuming parameters typical of current IFS
surveys, we confirm that there are strong positive correlations between
$\lambda_R$ and measurement radius, and strong negative correlations between
$\lambda_R$ and size of the PSF, for late-type galaxies; these biases can be
reduced using a recently proposed empirical correction. Once observational
biases are corrected for, we find that $\lambda_R$ provides a good
approximation to $\sim \sqrt{3}/2 \; \lambda'(\rm R_{\rm eff})$, where
$\lambda'$ is evaluated for the galactic stellar component within 1 R$_{\rm
eff}$.
