Population diagram of CH₃OH toward Sgr B2(N1S). The observed data points are shown in various colors (but not red), as indicated in the upper right corner of panel a, and the synthetic populations are shown in red. No correction is applied in panel a. In panel b, the optical depth correction has been applied to both the observed and synthetic populations, and the contamination by all other species included in the full model has been removed from the observed data points. The purple line is a linear fit to the observed populations (in linear-logarithmic space).