the diagnostic. In this instance, the degree to which the average strong line value of a given stack is representative of the galaxies within that stack is less meaningful than with other diagnostics. This is a primary concern when stacking galaxies (see Footnote 14 of @Salim14 for an example of how binning can lead to the wrong impression).

An additional concern with strong line abundance diagnostics is the effect of ionization parameter variations on the diagnostic ratios [@Kewley02; @Steidel14]. The ionization parameter $\Gamma$ is given by $$\Gamma \equiv \frac{\Phi}{n_H} \approx \frac{\Phi}{n_e}$$ where $n_H$ is the number density of hydrogen atoms and $\Phi$ is the density of hydrogen ionizing photons. Changes in the ionization parameter can be due to either variations in the temperature of the ionizing continuum (i.e. a galaxy composed of systematically hotter stars than average) and/or variations in the physical conditions of star forming regions (i.e. higher stellar densities and/or lower gas densities than average). In order to eliminate these biases, it would be advantageous to use a diagnostic that is insensitive to ionization parameter variations [e.g. N2O2, @Kewley02], though our choice of ${\hbox{$\Delta\log$(SSFR)}}$ as a second parameter should at least somewhat account for differences in ionization parameter 