bonds in a molecule (Group 4) does not generalize well to bonds with similar properties like >NH. If one wants to discuss molecular polarity, one should use higher-order parameters such as dipole (Group 2), which are determined as dependent variables of the various functional groups. Therefore, an effective variable selection strategy for small datasets is to choose parameters likely to generalize well (i.e., those from smaller group numbers) as the basis, then add lower-order variables as correction terms.