Plots of χ² in the EPF analysis of ¹³CO lines. The analysis used three T_kin temperatures, as listed above each column of frames. The cloud model is an isothermal BE sphere with M = 10 M_⊙ and T_kin = 20 K. First row shows the analysis of the J = 1 − 0 lines, and each subsequent row adds one further rotational transition. The contours are drawn at χ² levels of 1 (red), 2 (cyan), 10 (magenta), and 100 (blue contour). In order of increasing size, the circles indicate the reference density and column density values towards the model centre, averaged over a FWHM = R₀/3 beam (used for the synthetic observations), and averaged over a FWHM = R₀ beam.