Distribution of the spectral index α_ν of z > 2 of SDSS-DR7 quasars (red histogram) re-observed by BOSS (black histogram). The spectral index was measured using the rest frame wavelength ranges 1450−1500, 1700−1850 and 1950−2750 Å. As seen already from the composite spectrum shown in Fig. 5, the spectral indices measured using SDSS-DR9 quasar spectra are bluer than those obtained using DR7 spectra (see Sect. 7.2).