Host galaxy dust extinction in the optical/UV [E(B − V)_HG] as a function of the X-ray column density (N_H). Upper limits in E(B − V)_HG are shown as downward arrows. The solid black line shows the weak correlation between the two quantities if we exclude the upper limits (Sect. 7.2.2, Table 2) and the shaded gray region represents the uncertainty on the regression relation. The red points are the median E(B − V)_HG values (including upper limits) in bins of N_H and the dashed black line is the linear fit using survival analysis to include the contribution from all upper limits.