Difference of oxygen (left) and sulfur (right) total abundance derived by the direct method and the theoretical strong-line method versus the total abundance derived by the direct method. The horizontal dashed black lines demonstrate 12 + log(O/H)_direct = 12 + log(O/H)_StrongLine and 12 + log(S/H)_direct = 12 + log(S/H)_StrongLine. The horizontal dashed red lines represent the median values of metallicity differences. The gray error bars show the uncertainties of the direct method. Data points are color-coded by the ionization parameters of their bins.