Histograms of log (νL_ν(20 μm) /νL_ν(30 μm)) for PG QSO (top panel, broad orange bars), S1 (top panel, narrow red bars), SB (middle panel, broad green bars), S2 (middle panel, narrow yellow bars), LINER1 (bottom panel, narrow purple bars), and LINER2 (bottom panel, broad pink bars). The mean values and one standard deviation over the mean for each class of objects are shown with large crosses (with the same colour code as the histogram), continuous lines for PG QSO, SB, and LINER2, and dashed lines for S1, S2, and LINER1. The grey area of the plot shows the range of values expected for AGN according to the models given by Nenkova et al. (2008). The orange and yellow vertical lines show the same ranges but for inclination angles of i = 0° and i = 90°, assuming that these values are representative of face-on and an edge-on AGN (see text).