However, values from different authors sometimes differ by several pK a units, due to inconsistencies in the literature regarding experimental determination of pK a values. Such values for the same molecule can deviate by more than 10 pK a units. In particular, acid dissociation constants in non-aqueous solvents are often measured relative to each other and then anchored to the pK a of a reference compound. Inconsistent choice of reference compounds can lead to high errors (for instance, two different pK a values for a common anchor compound in acetone lead to pK a values differing by nearly 3 pK a units). Additional errors are introduced from inconsistent treatment of ion pairing, particularly in poorly-screening solvents; homoconjugation, in which neutral acids form hydrogen-bonded adducts with their corresponding conjugate anions; and impurity of solvent (e.g., water content). These errors alone can lead to differences of several pK a units. One pK a unit propagates to 1.4 kcal mol , so such problems in data quality can lead to very large errors in reported free energies.