Density distribution of the data (gray) and the selected samples (colored) in refined optical diagnostic (P₁–P₂) diagram. The blue grids are the photoionization models for SF regions, and the gray grids are the photoionization models for AGNs; each cross corresponds to a unique pair of metallicity and ionization parameter. Both grids are applied a cut so that only the parts that within the middle 98% of the total data along the hidden P₃ axis are shown. The definitions of P₁ and P₂ are in the text. The demarcation line corresponds to f_{S F} = 0.90, which means the maximum contamination to Hα from AGN-ionized regions is 10%.