The [O IV] 25.9 μm/[Ne II] 12.8 μm ratio plotted against the EW of the 6.2 μm PAH feature for optically identified star-forming (blue stars), composite (black disks), and AGN (green squares) galaxies. The green lines represent the average AGN fractional contribution to a galaxy’s emission, based on the [O IV]/[Ne II] ratio (Sturm et al. 2002). The blue lines denote the average starburst fractional contribution determined from the EW of the 6.2 μm feature (Armus et al. 2007). The red circle highlights galaxies identified as star-forming based on our diagnostic tool. The uncertainties associated with each measurement are 1σ error.