[S ii]/[S iii] versus [O ii]/[O iii] in logarithm units compared with a grid of WM-Basic models at Z = 0.2 Z_⊙. The solid lines represent models with the same effective temperature as indicated. Circles show H ii regions with reliable line emission measurements and triangles regions with upper limit intensities. Filled symbols represent data from our own sample (observed regions) while open symbols represent regions taken from the literature.