1. INTRODUCTION  
2. OBSERVATIONS  
2.1. The MOSDEF Survey  
2.2. Observations and Reduction  
2.3. Measurements and Derived Quantities  
3. ELECTRON DENSITIES  
3.1. Methods  
3.2. Sample  
4. IONIZATION PARAMETER  
4.1. Definition of the Ionization Parameter  
4.2. Sample Selection  
4.3. $O_{32}$ and Global Galaxy Properties  
4.4. $O_{32}$ and Metallicity  
5. DISCUSSION  
5.1. Is the Ionization Parameter $O_{32}$ Relation Redshift Invariant?  
5.3. Additional Evidence for the Existence of the Metallicity-Ionization Parameter Anti-correlation  
5.4. Nitrogen Abundance and the Cause of the [N II] BPT Diagram Offset  
5.5. Diffuse Ionized Gas and the Interpretation of Global Galaxy Spectra  
6. SUMMARY