  We present a comprehensive model to predict the rate of spectroscopic
confusion in HI surveys, and demonstrate good agreement with the observable
confusion in existing surveys. Generically the action of confusion on the HI
mass function was found to be a suppression of the number count of sources
below the `knee', and an enhancement above it. This results in a bias, whereby
the `knee' mass is increased and the faint end slope is steepened. For ALFALFA
and HIPASS we find that the maximum impact this bias can have on the Schechter
fit parameters is similar in magnitude to the published random errors. On the
other hand, the impact of confusion on the HI mass functions of upcoming medium
depth interferometric surveys, will be below the level of the random errors. In
addition, we find that previous estimates of the number of detections for
upcoming surveys with SKA-precursor telescopes may have been too optimistic, as
the framework implemented here results in number counts between 60% and 75% of
those previously predicted, while accurately reproducing the counts of existing
surveys. Finally, we argue that any future single dish, wide area surveys of HI
galaxies would be best suited to focus on deep observations of the local
Universe (z < 0.05), as confusion may prevent them from being competitive with
interferometric surveys at higher redshift, while their lower angular
resolution allows their completeness to be more easily calibrated for nearby
extended sources.
