Relation between the [O II]λ3727 effective wavelength λ_eff and the R[SII] intensity ratio for a sample of HII regions. Red circles refer to a vetted subsample in which the uncertainty in the [S II]λλ6731,6717 doublet ratio R is less than 10%. Best fitting lines are obtained with the bisector method (full sample, black line) and with an unweighted least square fit (vetted sample, blue line). Typical errors are δλ_eff ≈ ±0.15 Å for the effective wavelength and δR[SII]   ≈   ± 0.15 for the [S II]λλ6731,6717 ratio.