Correlating f_ion with the three input parameters (T_e([Ν ii]), 12+log(O/H), and [Ο i]/Ηα) for each H ii region in the CHAOS sample. Each of the four galaxies is shown with separate symbols, and the Spearman’s rank correlation coefficient (ρ) and its significance (p) are shown in each figure. The f_ion variations correlate most strongly with changes in [Ο i]/Ηα, but show a weaker correlation than the trends with [S iii]/[S ii] in Fig. 1.