Model comparison on the basis of Δ log Z between pairs of models, i.e., 1) the single-powerlaw (pw) model vs the APEC model; 2) the double-powerlaw (pw+pw) model vs the single-powerlaw model; 3) the powerlaw + blackbody (pw+bb) model vs the single-powerlaw model; 4) the powerlaw + blackbody model vs the double-powerlaw model. The empty histogram indicates all the sources and the filled histogram corresponds to the subsamples of bright sources with at least 20 net counts in 0.2–5 keV band. In the first panel, the blue and orange colors indicate AGN and stars respectively. In the second and third panels, the red color indicates the subsamples of AGN with a median logN_H above 21.5. In the last panel, the blue and magenta colors indicate sources with log Z_apec − log Z_pW below and above 1.3 respectively.