Comparison of the intra-band spectral index (α_LOFAR) with the NVSS to LOFAR spectral index (α_LOFAR-NVSS). The colour of the dots is proportional to the deviation from the (black solid) line where the spectral indices are equal. The errors are plotted as faint grey lines and indicate the 95 per cent confidence interval. The median of the distributions for all the sources are shown as grey dash-dotted lines. The median of the distribution of α_LOFAR for the sources that were matched to NVSS sources is shown as a dotted line. The orange dashed line shows the total least squares fit to the distribution (see text). Upper panel: histogram with the distribution of α_LOFAR-NVSS. Right panel: histogram with the distribution of α_LOFAR for the cross-matched sources (blue) and for the total sample over-plotted (green).