Comparison of the cloudy modeled f_ion measured from [Ο i]/Ηα with what is derived using a fit based on [S iii]/[S ii] (as in Fig. 15, right). Left-hand panel: depends only on x = log₁₀([S iii]/[S ii]) while the right-hand panel includes a secondary dependence on age as traced by the EW of Hα, as y = log₁₀(EW(Hα)). Both plots are colored by the age of the ionizing stellar cluster, and correcting for this secondary dependence on age significantly improves the agreements between the fit and the value for f_ion in the cloudy models.