Maximum observed velocity υ_max (panel a), minimum observed velocity υ_min (panel b), central velocity υ_c (panel c), and BAL width Δυ (panel d) distributions for the P₄ sample (upper histogram in each panel) and for the main sample of sources with BAL troughs (lower histogram in each panel). Results of the K-S test performed on each pair of cumulative distributions are reported in each panel: D is the maximum distance between the two cumulative distributions, and P is the probability of obtaining a higher D value assuming that the two datasets are drawn from the same distribution function.