Results of stacking spectra per soft-excess strengths. Top panel: unfolded stacked XMM-Newton/PN and Swift/BAT spectra for the four groups of different q (normalized to the spectrum with a lower q value, at 15 keV). The q value is increasing for stacked spectra from black circles (Group 1) to red squares (Group 2), to green triangles (Group 3) and to blue dots (Group 4). Bottom panel: ratios of the stacked XMM-Newton/PN and Swift/BAT spectra. The ratio of stacked spectra of higher q (Group 4) over lower q (Group 1) is represented in blue points. The ratio of stacked spectra from Group 3 over Group 1 is plotted in green triangles. The ratio of Group 2 over Group 1 is in red squares.