Joint distribution of the column density and peak volume density, for all the lines of sight of all the clouds in the sample. The trend of this distribution has been fitted as a power law in three column density regimes: for the entire column density range present in the data (solid line), for the low-density molecular gas defined as A_V = 1–8 mag (dashed line), and for the dense gas defined as A_V > 8 mag (dotted line). The RMS scatter around the best fit relation is represented by the shaded areas.