  We present a comparative study between the results of most hydrodynamic
simulations of the common envelope binary interaction to date and observations
of post common envelope binaries. The goal is to evaluate whether this dataset
indicates the existence of a formula that may predict final separations of
post-common envelope systems as a function of pre-common envelope parameters.
Some of our conclusions are not surprising while others are more subtle. We
find that: (i) Values of the final orbital separation derived from common
envelope simulations must at this time be considered upper limits. Simulations
that include recombination energy do not seem to have systematically different
final separations; these and other simulations imply $\alpha_{\rm CE} <
0.6-1.0$. At least one simulation, {applicable to double-degenerate systems},
implies $\alpha_{\rm CE} < 0.2$. (ii) Despite large reconstruction errors, the
post-RGB observations reconstructed parameters are in agreement with some of
the simulations. The post-AGB observations behave instead as if they had a
systematically lower value of $\alpha_{\rm CE}$. The lack of common envelope
simulations with low mass AGB stars leaves us with no insight as to why this is
the case. (iii) The smallest mass companion that survives the common envelope
with intermediate mass giants is 0.05-0.1~\ms. (iv) Observations of binaries
with separations larger than $\sim$10~\rs, tend to have high $M_2/M_1$ mass
ratios and may go through a relatively long phase of unstable Roche lobe mass
transfer followed by a weakened common envelope (or with no common envelope at
all). (v) The effect of the spatial resolution and of the softening length on
simulation results remains poorly quantified.
