Histograms of depletion times for the dense gas in the whole sample of galaxies (solid line) and in the populations of LIRGs/ULIRGs (gray filled) and of normal galaxies (hatched). The depletion times in the left-hand panel are calculated assuming the same FIR-to-SFR and HCN-to-mass conversion factors for the two populations. In the middle panel, the two conversion factors are modified (see Sect. 6), whereas in the right-hand panel only the HCN-to-mass factor is changed.