Results of the simulations of 1T, two-N_H spectra (~18 000 counts for N_H = 0) fitted with 2T, one-N_H models. The black contours in the upper panel show the average null probability as a function of the fraction of obscured emission measure, f_abs, and the column density of the obscuring material, N_H. The middle and lower panels show contour plots of the mean unabsorbed flux (in the 0.5–8.0 kV band, in units of 1.3 × 10^-13 s^-1 cm^-2), and of the mean N_H (units 10²² cm^-2), both obtained from the spectral fits. The two white contours in the upper panel are repeated from the middle panel and show the loci corresponding to a 50% and 70% reduction in measured flux.