  We present the results of a set of mock experiments aimed at quantifying the
accuracy of results derived from HI spectral line stacking experiments. We
focus on the effects of spatial and spectral aperture sizes and redshift
uncertainties on co-added HI spectra, and by implication on the usefulness of
results from HI line spectral stacking experiments. Using large spatial
apertures to extract constituent galaxy spectra yields co-added spectra with
high levels of contamination and with relatively low signal-to-noise ratios.
These properties are also affected by the size of the spectral aperture as well
as HI redshift uncertainties of galaxies. When redshift uncertainties are high,
S/N decreases while the contamination level remains roughly constant. Using
small spectral apertures in the presence of large HI redshift uncertainties can
yield significant decreases in S/N without the expected decrease in amount of
contaminant flux. Our simulations show that a co-added spectrum rarely yields
an accurate measure of the total HI mass of a galaxy sample. Total mass is
generally over/under-estimated for large/small spatial apertures, regardless of
spectral aperture size. Our findings strongly suggest that any co-added HI
galaxy spectrum needs to be fully modelled in the ways presented in this paper
in order to apply accurate corrections for flux contamination and derive
realistic uncertainties in total HI galaxy mass. Failing to do so will result
in unreliable inferences of galaxy and cosmological parameters.
