Evolution of the radio spectral index, , with redshift, as inferred from our stacking analysis. Red circles and error bars correspond to the median and interquartile range observed in our study in regions of the parameter space not affected by incompleteness (see values and green lines reported in Fig. 5). Stars show results from Ivison et al. (2010a), while triangles show results from Bourne et al. (2011). Both studies are based on a stacking analysis, but Ivison et al. (2010a) applied it to a sample of 24 μm-selected galaxies while Bourne et al. (2011) applied it to a stellar-mass-selected galaxy sample. The light grey region shows the range of values (i.e. 0.7 ± 0.4) observed by Ibar et al. (2009) in a population of sub-mJy radio galaxies. The dark grey region presents the range of values (i.e. 0.75 ± 0.29) observed in a population of SMGs at zs2 (Ibar et al. 2010). Note that the ranges of values observed in Ibar et al. (2009, 2010) correspond to the intrinsic dispersion observed in these galaxy populations and not to measurement errors on the mean radio spectral indices.