N_e(O ii) versus N_e(S ii). Black stars: VLT-UVES; filled blue circles: FORS observations (medium resolution). Purple triangles: data from García-Rojas & Esteban (2007), green asterisks: data from Esteban et al. (2009). The dashed blue line connects points of equal densities. The lower panel shows the error bars and regression lines and their 1σ alternatives for all data (purple lines) and for the VLT-only data (black lines). (A color version of this figure is available in the online journal.)