Relation between the N3O3 parameter and the abundance ratio N/O for different models at a fixed 12 + log(O/H) = 8.9. The models are compared with the N/O abundances derived from the optical lines for a sample of local star-forming galaxies with IR spectroscopic observations, including dwarf galaxies (purple circles), nearby starbursts (red triangles), and local (U)LIRGs (green squares). Three galaxies at high-z with detected [N III] 57 μm and [O III] 52, 88 μm are also included (orange squares).