D/H ratio of double deuterated molecules as a function of the D/H ratio of the corresponding simple deuterated molecules, including statistical corrections in both cases (see text). Only starless cores and Class 0 sources are included, for direct comparison. The dataset is colour-coded according to the corresponding molecule, as indicated in the legend. In the case of methanol, the single deuterated molecule is CH₂DOH. For both ratios, circles represent data obtained with single-dish observations for both ratios, while triangles represent data obtained with interferometric observations. In the case of H₂S, open symbols are the upper limits, corresponding to the sources of this work, where only HDS was directly detected. The red line shows the statistical value if the D atoms were statistically distributed in the molecules formed in the grain surfaces (see text).