Panel a: nitrogen to oxygen line ratio N₂/R₂ as a functionof the N₂ line intensity for the SDSS fibre spectra (catalogues SDSS-A and SDSS-B), for the spaxel MaNGA spectra, and for the slit spectra of H II regions in nearby galaxies. The symbols represent the mean values for objects in bins of 0.05 dex in logN₂. Panels b,  c, and d: same but for the sulphur to oxygen line ratio S₂/R₂, for the nitrogen to sulphur line ratio N₂/S₂, and for the excitation parameter P, respectively.