Relation between the rest-frame monochromatic X-ray (2 keV) and UV (2500 Å) fluxes for quasars in the redshift range z = 3.0−3.3. The colour code is the same as that adopted in Fig. 2 for the different sub-samples defined in Sect. 2.3. Grey points represent the other sources in the same redshift range from the cleaned quasar sample of RL19 (30 objects). The red line is the regression best fit for the z = 3.0−3.3 RL19 sources only. Slope and dispersion values are reported with the residuals from the best fit in the bottom panel (from which X-ray weak, in brown, and weak candidates, in orange, are omitted for clarity).