Oxygen abundance versus the N2 index for the H ii complexes within CALIFA galaxies. We plot the CALIFA-ONS H ii regions as a blue density contour plot. Our new N2 calibration (see Fig. 4) is plotted with a green solid line. Red lines are used to show the CALIFA-ONS best linear fit (solid) and its 2σ prediction intervals (dot-dashed) with its applicability interval (from N2 = −1.6 to −0.2, vertical dashed lines) (see text for details on these fits). In this case we obtain 12 + log (O/H) = 8.667 +0.455   ×   N2. The blue cross and bar in the top right corner of the plot indicate the average random errors (cross) of the CALIFA-ONS data points and the typical systematic errors (vertical bar) associated with the measurement of ionized-gas oxygen abundances.