Rules and Precedents as Complementary Warrants

This paper describes a model of the complementarity of rules and precedents in the classiication task. Under this model, precedents assist rule-based reasoning by operationalizing abstract rule antecedents. Conversely , rules assist case-based reasoning through case elaboration, the process of inferring case facts in order to increase the similarity between cases, and term reformulation, the process of replacing a term whose precedents only weakly match a case with terms whose precedents strongly match the case. Fully exploiting this complementarity requires a control strategy characterized by impartiality, the absence of arbitrary ordering restrictions on the use of rules and precedents. An impartial control strategy was implemented in GREBE in the domain of Texas worker's compensation law. In a preliminary evaluation, GREBE's performance was found to be as good or slightly better than the performance of law students on the same task. In a variety of domains, such as law, both general rules and speciic precedents are useful for performing classiication | the task of assigning a given input, or case, to a category and explaining the assignment. This section explains the complementarity of rules and precedents for performing classiication and the disadvantages of arbitrarily restricting the order in which they can be combined. A case is classiied as belonging to a particular category by relating its description to the criteria for category membership. The justiications, or warrants Toulmin, 1958], that can relate a case to a category, can vary widely in the generality of their antecedents. For example, consider warrants for classifying a case into the legal category \negligence." A rule, such as \An action is negligent if the actor fails to use reasonable care and the failure is the proximate cause of an injury," has very general antecedent terms (e.g., \breach of reasonable care"). Conversely, a precedent, such as \Dr. Jones was negligent because he failed to count sponges during surgery and as a result left a sponge in Smith," has very speciic antecedent terms (e.g., \failure to count sponges"). Both types of warrants have been used by classiication systems to relate cases to categories. Classiication systems have used precedents to help match the antecedents of rules with cases. Completing this match is diicult when the terms in the antecedent are open-textured, i.e., when there is sig-niicant uncertainty whether they match speciic facts Gardner, 1984, McCarty and Sridharan, 1982]. This problem results from the \generality gap" separating abstract terms from …