Left: average spectra for PG QSOs (orange), S1s (red), AGN-dominated S2s (yellow), and AGN-dominated LINERs (purple). Right: average spectra for SBs (green), SB-dominated S2s (yellow), and SB-dominated LINERs (purple). We also show one standard deviation as a shaded region using the same colours. Each spectrum is also marked with different symbols at 27 μm for clarity of the plot: PG QSOs (orange star), S1s (red square), SBs (green triangle), S2s (yellow square), and LINERs (purple circle). The average spectra are scaled to the flux at 15 μm.