Comparison between O/H derived from HCm and from the empirical calibration of the O3N2 parameter given by Pérez-Montero & Contini (2009). The blue solid lines represent the 1:1 relation. The left panel has a colour scale for the [O ii]/[O iii] ratio and the right panel for N/O. For the sake of consistency, the number of points in left panel is lower because [O ii] lines were not used for the derivation of O/H. The upper left crosses represent the standard deviations of the residuals to the direct method of both methods as compared to the direct method carried out by Pérez-Montero & Contini (2009) and Pérez-Montero (2014).