Top panel: the distribution of S20/S3.6 in the general source population, in the sample of HzRG by Seymour et al. (2007), and in our IFRS sample. The IFRS clearly occupy a different regime than the general population, and tend to overlap more with the HzRG. Bottom panel: the histogram of the IFRS S20/S3.6 ratios as in the upper panel, shifted to the right by log(5) = 0.7. This takes into account that Norris et al. (2010) found no IR counterparts for IFRS in a stacking analysis with a 5 times higher sensitivity. On average the IFRS then have a S20/S3.6 which is about two times higher than that of the HzRG.