Sensitivity of the model results to the fitted lines. We show histograms of the reduced χ² () and best-fitting parameters (n_H, log U, cov_PDR) for the entire sample of galaxies. The results from the default grid of models and the default set of lines are in black, and the results are in color if the set of fitted lines is changed, that is, when one spectral line is excluded or when the H₂ lines included or when we consider a small, fixed set of lines to fit ([Ne III]₁₅, [O III]₈₈, [C II]₁₅₇, and L_TIR). The vertical dotted lines indicate the mean values. Each histogram is offset vertically by a value of 20. See Sect. 5.1 for details.