and therefore similar values of the other parameters that impact the connection between strong line ratio and metallicity. We consequently only stack galaxies with similar stellar masses and star formation rates to minimize the dispersion in galaxy properties in each stack. Good support for this approach comes from an investigation of stacking by @Andrews13. They compared electron temperatures and abundances for galaxies with individual auroral line detections to stacks of the same sample of galaxies and found good agreement within the measurement uncertainties.

We have performed a bootstrap analysis as an additional validation of this approach. For this analysis we chose four bins of different star formation rates at the same stellar mass. We resampled each bin 100 times and processed them with our analysis pipeline to derive the metallicity. We found the median of the bootstrap metallicity distribution agreed well with the stack value for each bin. The spread in the metallicity distribution ($\sim 0.15$ dex) was somewhat larger than the formal metallicity uncertainties, but smaller than the variations in the strong line ratios at fixed stellar mass and star formation rate ($\sim 0.2$ dex).

We have chosen to use both stellar mass and star formation rate because there 