Variability vs. (a) black hole mass, (b) redshift, (c) Eddington ratio, and (d) continuum luminosity at 2500 Å. The blue dots and lines are for BWB quasars, while the red dots and lines are for RWB ones. The results of Spearman correlation analysis (correlation coefficient and confidence level) are shown in each panel for BWB and RWB objects. In the side panels, histograms of the four parameters are shown for both BWB and RWB quasars, along with their median values. The average probabilities p are also given for a χ² test performed 100 times that compares the randomly selected RWB samples with BWB quasars.