Histograms showing the distribution of AGN and shock-sensitive emission-line ratios (from left to right: [N ii]λ6583/Hα, [S ii]λλ6717, 6731/Hα, and [O i]λ6300/Hα) for barred and unbarred galaxies separately. All galaxies classified as non-AGN are included (see Sect. 4). The barred/unbarred distributions are only different for [N ii]λ6583/Hα, and therefore the source of the differences in that line ratio must originate from something other than shock ionisation and/or AGN contamination alone.