Comparison between the O/H abundances derived from HCm with the empirical calibration of the O3N2 parameter given by Pérez-Montero & Contini (2009). In this case HCm O/H values were calculated without a previous determination of N/O, but assuming the expected O/H-N/O relation. Density of points over the mean value are represented according to the colour bar. The red solid line represents the 1:1 relation. The upper left cross represents the standard deviations of the residuals of both methods as compared to the direct method carried out by Pérez-Montero & Contini (2009) and Pérez-Montero (2014).