Comparison of the difference in the values of the dust mass as computed from radiative transfer modelling of optical data and as computed from black-body fitting of Herschel datapoints, with various physical quantities: the (optically derived) mass of dust, the dust temperature, the absolute K-band magnitude and the dust scaleheight. The starred points correspond to two other well known edge-on galaxies not included in our sample: NGC 891 (blue) and NGC 4565 (green).