is good evidence that metallicity depends on star formation rate at fixed mass (Ellison et al. 2008; Mannucci et al. 2010; Lara-Lopez et al. 2010). In addition, we expect galaxies with different star formation rates at fixed mass may differ in other parameters (incident spectral shape, etc.). While the integral field study by @Sanchez13 did not find that metallicity depends on star formation rate at fixed mass, we emphasize that our decision to stack in both quantities is also motivated by how other physical parameters vary with star formation rate.

It is also well known that stellar mass and star formation rate are well correlated, a correlation known as the star forming main sequence [@Brinchmann04; @Salim07; @Noeske07; @Whitaker12; @Zahid12b; @Kashino13]. In order to characterize this dependence, @Salim14 showed that the parameter ${\hbox{$\Delta\log$(SSFR)}}$

$${\hbox{$\Delta\log$(SSFR)}}= \log({\rm SSFR}) - \left\langle \log({\rm SSFR}) \right\rangle_{\small{M_{\star}}}
\label{eq:dssfr}$$

is more effective than both SFR and SSFR at identifying low and high oxygen abundance outliers across a wide range in ${\hbox{$M_{\star}$}}$. The quantity $\left\langle \log({\rm SSFR}) \right\rangle_{\small{M_{\star}}}$ is the median $\log$(SSFR) of galaxies at ${\hbox{$M_{\star}$}}$. Thus ${\hbox{$\Delta\log$(SSFR)}}$ is defined relative to the star forming main sequence rather than an arbitrary value (e.g. 1 ${\hbox{$M_\odot$ yr$^{-1}$}}$).

Binning in ${\hbox{$\Delta\log$(SSFR)}}$ 