)O)=O, 14, 18) were over-represented by a factor of 22.36. We also calculated pair enrichment factors (PEFs), as described in Equation , for the set of molecules. The descriptor pair that showed the greatest degree of this dependence in the set is the iminohetarene & secondary , which yielded a PEF of 2,308, relative to a mean PEF of 49.83 for the set. Two other descriptor pairs yielded PEFs greater than 2,000: