Radio spectral index between the observed-frame frequencies of 1.4 and 3 GHz plotted as a function of redshift. The red up-pointing triangles show the lower limits. The cyan filled circles represent the median values of the binned data with the error bars showing the corresponding standard errors. The lower redshift bin contains 57 SMGs, while the higher redshift bin contains 58 SMGs. The large number of censored values allowed us to only halve the sample to apply survival analysis. The horizontal dashed line marks the median spectral index of , which was calculated using a survival analysis technique.