Comparison between the metallicity derived with the code HCm and different strong-line calibrations. Top: a combination of calibrations by Maiolino et al. (2008) and corrected McGaugh (1991) calibration. Bottom: calibrations by Pettini & Pagel (2004) and Marino et al. (2013). A histogram is shown in each panel displaying the difference between the metallicities estimated with each of the four strong-line methods and HCm. In the bottom part, the difference is plotted as a function of HCm values.