Simulated basic quasar sample (a)–c); small green dots), selected sample (c)–e); small blue crosses), and simulated “observed” quasar sample (f)–h); small red crosses). The selection is based solely on the luminosity limits of the SDSS sample (c); see also Fig. 4a). The thick diagonal lines in the lower right corner of panels e) and f) indicate the displacement for a 0.5 dex shift in M. The “observed” sample is identical with the selected sample but mass errors were involved in the latter. Panels f) to h) show correlations in the “observed” sample. Solid line in f): linear regression curve. See the text for a more detailed description.