Comparison between the observed molecular abundances (X, Table 10) of MF and DE (upper panel), MF and EC (middle panel), and DE and EC (lower panel). The sources analysed in this work are drawn with filled coloured circles, while literature ones (see Sect. 6.2.1 and Table F.1) with empty coloured circles. All abundances are relative to H₂. Error bars are shown whenever available. The solid black line corresponds to the linear best-fit to the data of the sources studied in this work, while the dashed grey line to the identity. The linear correlation coefficient between the two molecules (r) is also given.