Estimated distribution of the rest-frame Hydrogen column density for our high-z sample. The black, solid histogram corresponds to sources with a significant observed N_H (i.e. above the Galactic value) and corrected to rest-frame values. The hatched histogram shows our estimated number of unabsorbed sources. The blue histogram shows the corresponding distributions for the optically selected QSO by Yang & Shen (2023) included in our sample.