Left-panel: distribution of the O3N2 vs. R₂₃ line ratios for all the 1142 H ii regions with detected [O ii] λ3727 emission line. The image and contours show the density distribution of both parameters. The first contours is at the mean density, with a regular spacing of four times this value for each consecutive contour. The red-dashed line shows the best fit found using a 2nd-order polynomial function between the two considered parameters. The light-blue solid-circles indicate the median values of both parameters, with their corresponding standard deviations represented as error bars, for consecutive bins of 0.10 in ΔO3N2. The black solid line shows the relation expected between both parameters when they are used to derive the same oxygen abundance, assuming the Tremonti et al. (2004) fit for R₂₃ and the Pettini & Pagel (2004) calibration for O3N2. The dot-dashed blue line shows the same relation, if the oxygen abundance derived based on R₂₃ was 70% of the one derived using the Tremonti et al. (2004) fit. Central panel: similar plot for the distributions of the N2O2 line ratio vs. R₂₃, for the same H ii regions. In this case the light-blue solid-circles indicate the median values and corresponding standard deviations of the parameters for consecutive bins of 0.10 in ΔN2O2. Right panel: similar plot for the distribution of the N2 line ratio vs. O3N2. The blue solid circles indicate the median values of both parameters, with their corresponding standard deviations, for consecutive bins of 0.10 in ΔO3N2. The red dashed-line shows the best fit found using a 2nd-order polynomial function between both parameters. In the three panels the typical/median errors of the represented parameters are represented as a black error-bar.