Ensemble PSDs of quasars of all M_BH and λ_Edd, with different λ_rest (indicated with different colours). Power spectra of quasars with various M_BH and λ_Edd are normalized to log(M_BH) = 8.9 and log(λ_Edd) = 0.1 using Eqs. (9) and (10). Solid lines show the best-fit linear models to the data (best-fit slopes are in agreement with the best-fit results plotted in the top panel in Fig. 14).