Comparison between the SII diagram (left) and the OB-I diagram (right). In the OB-I diagram, the black line and the shaded region represent Eq. (2). In the SII diagram, the solid black line represents the separation by Kewley et al. (2001). Each contour represents 20% more of the sample, and the error bars represent the median error for each axis, for each classification. On the left is the OB-I classifications on the SII diagram; on the right is the SII classifications on the OB-I diagram. The top panels represent SF galaxies and bottom panels represent AGNs.