given ionic species can be computed, which can then be used to determine relative abundances.

The direct method is subject to some biases. Temperature fluctuations and gradients in regions produce a bias towards lower metallicities [@Peimbert67; @Kobulnicky99] . This bias also applies to integrated (as well as stacked) spectra of galaxies. Hotter regions have brighter auroral lines, which can bias the direct method toward higher electron temperatures and correspondingly lower metallicities. Additionally, the assumption of a Maxwell-Boltzmann electron energy distribution has recently come into question [@Nicholls12; @Dopita13]. If electron energies are instead well described by a $\kappa$-distribution, this may contribute to the well known temperature discrepancy problem [@Garcia-Rojas06; @Garcia-Rojas07; @Nicholls12; @Blanc15], although this is less of a concern for relative comparisons of direct method abundances. Even with the potential for these systematic effects, the direct method is widely regarded as the standard for nebular abundances.

In practice, dectecting the auroral lines (e.g. ${\hbox{[\ion{O}{iii}]~$\lambda$4363}}$) requires a significant investment of observational resources for even the brightest, most metal poor galaxies and regions. At present, most spectroscopy comes from low to moderate SNR, and direct method abundances are typically not practical.

In order to estimate the metallicities of galaxies without the use of 