Ratio of the observed and the true ¹³CO column densities for synthetic observations of Bonnor-Ebert spheres. There are three sets of curves that corresponds to cloud masses of 0.3, 1.0, and 3.0 solar masses (from right to left, colours red, green, and blue, respectively). The solid, dashed, and dotted lines refer to averaging of the data (observed intensity and true values) within 10%, 50%, and 100% of the cloud outer radius. The length of each line corresponds to a variation of the ¹³CO abundance between 0.3 and 3 times the default value of 10^-6.