Comparison of the estimated column density of ¹³CO on the Horsehead pillar data for different sets of species and/or a priori. In all cases T_kin, $n_{{\text{H}}_2}$, C_V and σ_V are assumed to be the same for all lines. Top: maps of estimated N(¹³CO). Table 4 lists the details of each studied case. Bottom: joint histograms of log N(¹³CO) estimations as a function of different sets of species and there associated a priori hypotheses, shown as the column and row labels. In each panel, the green, blue, and red lines show the identity function, and ratios of values within a factor 2 or 10, respectively. The top left and bottom right legends give the number of pixels used to compute the histogram, and the mean and standard deviation of the distance to the green line.