Comparison of the CRB precisions when combining the first two J transitions of one or two CO isotopologues. These reference precisions are plotted for different values of column densities (y-axes) and volume densities (x-axes). The values of the parameters that are fixed when generating the RADEX spectra are listed in the bottom row. The precision depends on the a priori knowledges that are taken into account by the estimator. These are listed in the second row. The first two columns show the precisions for a single CO isotopologue (¹³CO or C¹⁸O, while the last column shows the precisions when studying the ¹³CO and C¹⁸O isotopologues simultaneously. From top to bottom, the panels show the precisions on the ¹³CO and C¹⁸O column densities, kinetic temperature, volume density, and thermal pressure. The CRB precisions are color coded as listed in Table 8.