Distribution of spectral indices (for the sources present in the ERCSC at a given frequency in our sample with completeness of 80% or above). Here is shown as a dotted line, as a dashed line, and as a solid line. The region 2 ≤ α ≤ 4 is typical of thermal dust emission. In red we show the dusty sources, whereas in blue we show the synchrotron sources. The sources in all three samples (deep, medium, and shallow) are combined here. Note that, as expected, the 100 GHz, and 143 GHz samples are dominated by radio galaxies, whereas the 545 GHz and 857 GHz samples are dominated by dusty galaxies. At 217 GHz and 353 GHz we observe the transition between the two populations, with significant numbers of both types being present in the samples.