Upper panel: distribution of best-fit N_H values obtained for all the 180 sources at z >  0.4. The histogram in red shows the N_H distribution of absorption detected sources, while the blue histogram shows that of the upper limits. We note the limited but uniform sensitivity of our spectra to N_H due to the rest-frame bandpass (see text). Lower panel: box-and-whisker plots of N_H measurements for the lowz, midz and highz groups. Following the normal convention, the box represents the inter quatile range (the middle 50%), the bar in the middle represents the median value, and the whiskers show the range of the minimum and the maximum of the distribution.