Results for the radio AGN sample after adding offsets to spectral indices. (a)−(b) Comparison of spectral indices before and after adding the offsets described in Sect. 3.1. Cumulative distributions of ${\overline{\sigma }}_{[\mathrm{O}\mathrm{I}\mathrm{I}\mathrm{I}]}$ after reclassifying sources with the spectral index offset for the (c) low redshift and (d) high redshift sample. Colour maps showing the average ${\overline{\sigma }}_{[\mathrm{O}\mathrm{I}\mathrm{I}\mathrm{I}]}$ values and the fraction of disturbed sources in bins of L_1.4GHz and L_[O III], again after reclassifying sources with the spectral index offset. (e) PS and (f) NPS sources in the low-redshift (0.02 < z < 0.4) sample. (g) PS and (h) NPS sources in the high-redshift (0.4 < z < 0.8) sample.