Left hand panel: red asterisks show the best fitting values of A₃₀₀₀ and α_λ, as obtained for reddened quasars. Red asterisks surrounded by blue squares represent BAL quasars. The black filled square represents the corresponding mean value, and the error bars the standard deviations. The empty black triangle and the associated error bars denote the α_λ mean value and the 1σ dispersion in the case of unreddened quasars. Solid orange and dotted green curves represent the maximum value of A₃₀₀₀, such that, for a given bolometric luminosity L_bol, the observed flux at a given wavelength is greater than the flux detection limit in the Z-band and I-band, respectively. The bolometric luminosity has been varied in the range 10⁴⁷ < L_bol < 10⁴⁸ (curves from the bottom to the top). The orizontal black solid line represents the minimum extinction required for a quasar to be classified as “dust reddened”. Quasar spectra reddened by an extinction lower than this minimum value (gray shaded area) can still be fitted with a power law redder than the intrinsic one, and classified as “unreddened”. The black dashed line shows the effect of quasar color selection criteria on our results. High-z quasars are selected to have a blue color on the filters redwards of the dropout (z−J and i−z). This requirement can be translated into a maximum observed spectral slope (α_λ,max) which implies a maximum extinction A_3000,max affecting quasars. Clearly A_3000,max depends on the intrinsic slope α_λ of the quasar, and it is shown with the black dashed line for α_λ,max = −0.2, i.e. the minimum of the slope distributions in the SDSS quasar survey. Quasars reddened by an extinction higher than the dashed line (gray hatched region) are lost by the quasar color selection criteria. Right hand panel: trend of A₃₀₀₀ with redshift. The lines represent the maximum value of A₃₀₀₀ for which a reddened spectrum, for a fixed bolometric luminosity (L_bol = 10⁴⁸   erg   s^-1), is still detectable by the SDSS. The results have been obtained assuming a slope for the intrinsic continuum α_λ = −2.4 (dotted), α_λ = −1.6 (dashed). The horizontal black solid line and the gray shaded region have the same meaning of the ones in the left hand panel.