Population diagrams for CH₃ OH, ¹³CH₃OH, CH₃CN, ¹³CH₃CN, and CH₃¹³CN towards IRAS 4B. Observed data points are shown with teal circles as indicated in the top-right corner of the left panel while the modelled data points from Weeds are shown as orange squares. No corrections are applied in the left panels while in the right panels corrections for opacity and contamination by other molecules have been considered for the observed and modelled populations. The results of the linear fit to the observed data points are shown in the right panels.