the same. Figure \[fig:ref\_spec\_OII\] demonstrates the benefit of stacking. In the raw SDSS spectrum of a single galaxy (gray line), the weak auroral lines are undetectable. They become fairly evident after stacking (blue line). After removing nearby stellar continuum features (red line), the previously undetectable auroral lines are prominent features in the final spectrum (black line).

Choice of Stacking Parameters {#sec:justify_stack}
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Our goal is to derive improved strong line calibrations, so one of the parameters we use to assign galaxies to a stack is similar strong line ratios. However, the strong line ratios show considerable dependence on more parameters than just metallicity, such as incident spectral shape, ionization parameter, and gas density [@Dopita00; @Kewley02; @Dopita13]. For example, Steidel et al. (2014) demonstrated that variations in line ratios due to a factor of five change in metallicity could be reproduced with only a factor of two change in ionization parameter. Figure 2 clearly demonstrates that there is a substantial range in stellar mass and star formation rate at a constant value of the N2, O3N2, N2O2, or R23 strong line diagnostics.

As in @Andrews13, we assume that galaxies with similar stellar masses and star formation rates have similar physical conditions, 