A careful inspection of invalid imputations revealed some systematic problems associated with specific datasets. Applied to data derived from the USPTO database (Urnd, Udiff, Uunb) the rule base method produced uncertain predictions associated when {O : 1, Q : 0} being on the reactant side during rule application. Consider, for example the conversion of ethanol to ethane in Fig. , which is usually performed by dehydration and subsequent hydrogenation or by application of hydroiodic acid HI.