First ten principal components of a PCA applied to (i) z ~ 3 SDSS quasar spectra (black solid lines) over the range 1020−2000 Å and to (ii) z ≤ 1 HST quasar spectra (S05, red dashed lines) over the range 1020−1600 Å. The distributions of the coefficient associated to each component are shown in the right panel (grey histogram) together with their fit with a Gaussian (except for the first component for which the distribution is log-normal; thick black line).