LTE population diagrams (PD) of methyl formate (HCOOCH₃), ethanol (CH₃CH₂OH), and glycolaldehyde (CH₂OHCHO). For CH₂OHCHO, we complemented our dataset (blue diamonds) with the dataset of Taquet et al. (2015) (green diamonds). The results are plotted with (shaded) and without (full color) the millimeter dust obscuration factor. Left: column density-temperature reduced ${\tilde{\chi }}^2$ contour plots. The shaded and solid colored lines indicate the 1σ confidence ranges. The star marks the best-fit value in the two cases. Center: observed transitions corrected for the line opacity calculated at the best-fit values. The gray solid line refers to the best-fit of the data points (not corrected for dust obscuration). Empty markers indicate the points not corrected for line opacity, whose fit is the rotational diagram (RD). Right: line opacity as a function of the upper-state energy of the best-fit values.