Top: HUD HIMF (dashed green line and error bars) calculated via our ratio approach (see text) compared to the full α.70 HIMF (bluesolid line and error bars) and its Schechter function fit (dotted black line). We also show for comparison the Santa Cruz SAM HIMF for all galaxies (red solid line) and central UDG HIMF (dashed purple line). Bottom: fraction of ALFALFA HI sources that are HUDs with all appropriate weightings applied. Error bars in both plots are 1 − σ and only account for Poisson uncertainties. Bins that contain only one object have been removed.