Distribution of the SDSS SFG sample (circles) and local analogues (triangles) in the log U vs log OH plane, according to the likelihood procedure with the Feltre et al. (2016) models described in Sect. 2.3. In each panel, we also show (red dashed line), the region of the parameter space that we decided to exclude, a posteriori, from the same photoionization models. The upper panels show the variation in the distribution of the sources by considering models with different electron densities n_H. The middle panels show different dust-to-metal ratios ξ, the bottom panels different C/O abundance. The excluded region is never significantly populated by any source in all the panels.