Redshift evolution of the mean flux from SDSS DR7 quasar spectra. Three different estimates of the continuum are assumed (Sect. 2.1): (i) an extrapolation of a power-law fit (see Sect. 2.2; red triangles); or a prediction (Eq. (4)) using the output of a principal components analysis of (ii) SDSS spectra at z ~ 3 as described in Sect. 3 (blue squares) or of (iii) HST spectra at z ≤ 1 (S05; orange diamonds). Error bars are computed from bootstrapping and are at the 3σ level. A change in the evolution can be noticed at z ~ 3 in the sense of a steeper slope at high redshift. A featureless evolution (fitted from z ≤ 3 points using z ~ 3 PCA measurement) is shown for guidance.