Spectral stacks of the z ≃ 3 quasars. Top panel: mean observed quasar spectra for the X-ray normal (N, blue) and X-ray weak plus X-ray-weak candidates (W+w, red). These composites are estimated from a stack of the SDSS spectra (23 N and 10 W+w spectra), each normalised to unit flux at 1450 Å with uncertainties from bootstrap analysis (shaded area). The UV continuum slopes for the N and W+w composites are shown with the blue and red lines, respectively. Bottom panel: zoom-in of the C IV emissionf line for the N and W+w composites. The velocity shift from the rest-frame transition wavelength (1549 Å) is shown on the top x axis. In both panels, the ratio of the N over the W+w mean spectra is also shown at the bottom with a black line.