Distribution of the differences between the observed α_ox and the value predicted from the L_X–L_UV correlation for all the quasars in the z ≃ 3 sample. The dashed and solid curves are the best-fitting normal distributions for the core subset of the 25 radio-quiet sources (plotted in blue), where the peak position is left free to vary or forced to be at Δα_ox = 0. The dotted vertical line marks the one-sided 99% probability for the latter curve. Irrespective of the exact assumptions (see Sect. 5), a substantial fraction of objects clearly fall in the “X-ray weak” tail. The hatched grey distribution refers to “sample B” of Gibson et al. (2008).