Distribution of total flux in the 21 cm line as a function of line width for the certain detections (stars) and uncertain ones (circles). Filled and empty symbols show detections with and without an optical counterpart (OC), respectively, with the different shade fill segregating the spectroscopically confirmed and the potential optical counterparts. The dashed black line shows the ALFALFA reliability limit (Saintonge 2007) at 6.5σ. [Link]