Wide band (wb) SED. a) Shen catalogue with z< 2 (yellow solid curve: median, green shaded area: 1σ errors) and WLQ comparison sample (black dotted curve), normalised at 3000 Å. Over-plotted (blue): SDSS quasar composite spectrum from Vanden Berk et al. (2001). The horizontal lines indicate the wavelengths intervals covered by the different photometric bands. The inset shows the wb SED of all SDSS quasars (solid green) and of the comparison sample (black dotted) in the wavelength range 1300Å −3000 Å in linear scale, as well as the SDSS quasar composite spectrum smoothed with a 300 Å boxcar (blue dashed). b) WLQ-EWS sample (red solid curve) and comparison sample (black dotted curve and green shaded area). Over-plotted (blue): SDSS WLQ composite spectrum from Fig. 3. c) as b), but for the entire WLQ sample instead of the WLQ-EWS subsample. d) as b), but the comparison sample SED is boosted by a power-law component (plc) with α_λ = −1.7 and re-normalised, the green shaded area indicates the 1σ errors of the WLQ sample, and the SDSS composite spectrum was omitted for the sake of clarity.