Population diagram of CH₃OH, ν= 0, ν_t = 1, and ν_t = 2 toward Sgr B2(N2). Only the lines that are clearly detected, do not suffer too much from contamination from other species, and have an opacity below 2 are displayed. The observed data points are shown in black while 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 from all other species included in the full model has been removed from the observed data points. The straight line is a linear fit to the observed populations (in linear-logarithmic space). The rotational temperature derived in this way is reported in Table 6.