Distributions of the BEST sample spectral parameters. The BAND parameter populations are shown with blue dash-dotted lines, SBPL with green dash-double dotted lines, COMP with red dashed lines, and PL with purple solid lines. The overall populations (ALL) are shown with gray solid histograms. Top left panel: distributions of α. Top right panel: distributions of β. Bottom left panel: distributions of E_p. Bottom right panel: distributions of E_b.