Performance of the standard X_CO method to predict the column density from the ¹²CO (1–0) integrated line intensity. Top: spatial distribution of the observed and predicted H₂ column density (left and middle panels), and of the ratio of the predicted column density over the observed one (right panel). Bottom: joint histogram of the predicted column density as a function of the observed one (left panel), and histogram of the ratio of the predicted column density over the observed one on a logarithmic scale. These results are computed on the Horsehead pillar, i.e., the test set.