First ten principal components obtained from a PCA of the QSO spectrum redwards of the Lyman-α emission line, for (i) the z ~ 3 SDSS quasar spectra (black solid lines) and (ii) the z ≤ 1 HST quasar spectra (S05, red dashed lines). The distributions of the coefficient associated to each component are shown in the right panels (grey histogram) together with a Gaussian fit (except for the first component for which the distribution is log-normal; thick black line).