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We examine how much information measured broad-line widths add to virial BHmass estimates for flux limited samples of quasars. We do this by comparing theBH mass estimates to those derived by randomly reassigning the quasarbroad-line widths to different objects and re-calculating the BH mass. For 9000BH masses derived from the H_beta line we find that the distributions oforiginal and randomized BH masses in the M_BH-redshift plane and theM_BH-luminosity plane are formally identical. A 2D KS test does not find adifference at >90% confidence. For the MgII line (32000 quasars) we do findvery significant differences between the randomized and original BH masses, butthe amplitude of the difference is still small. The difference for the CIV line(14000 quasars) is 2-3sigma and again the amplitude of the difference is small.Subdividing the data into redshift and luminosity bins we find that the medianabsolute difference in BH mass between the original and randomized data is0.025, 0.01 and 0.04 dex for H_beta, MgII and CIV respectively. The maximumabsolute difference is always <=0.1 dex. We investigate whether our results aresensitive to corrections to MgII virial masses (e.g. Onken & Kollmeier 2008).These corrections do not influence our results. Moreover, the mass residual -Eddington ratio correlation discussed by Onken & Kollmeier is more directlyattributable to the slope of the relation between H_beta and MgII line width.Our results imply that the measured quasar broad-line velocity widths providelittle extra information, after allowing for the mean velocity width. In thiscase virial estimates are equivalent to M_BH L^alpha, with L/L_edd L^(1-alpha)(with alpha~0.5). This leaves an unanswered question of why the accretionefficiency changes with luminosity in just the right way to keep the meanbroad-line widths fixed as a function of luminosity. (abridged)

Do quasar broad-line velocity widths add any information to virial black hole mass estimates?