Absolute magnitude of quasars vs. redshift for the SDSS-based catalog of Schneider et al. (2010). Curves are computed for two limiting (K-corrected) apparent magnitudes m_B = 18.5 and m_B = 21.6. The shaded boxes identify the loci in the z − M_B planes of two volume-limited samples within the same luminosity limits: one between 2.2 ≤ z ≤ 2.5 as for the GTC sample of this paper, and a control sample at 0.05 ≤ z ≤ 0.6.