Histograms showing the number of sub-DLAs as a function of log N_H   I. Top panel: MIKE + ESI sample from O’Meara et al. (2007). The filled area represents the new sub-DLAs in MIKE + ESI sample and the unfilled area illustrates the ones overlapping with the combined sub-DLA sample. Middle panel: combined sub-DLA sample. Bottom panel: the solid black, dash-dotted gray and filled gray histograms show the contribution of sub-DLAs to the combined sub-DLA sample from the statistical EUADP, Péroux et al. (2005), and Péroux et al. (2003a) samples respectively.