Teaching Case-Based Argumentation Through a Model and Examples

ion hierarchies can be used in a straightforward way to compute the similarity of corresponding case features [Kolodner, 1993, p. 346-7]. To assess the degree of similarity of two different values, one finds their most specific common ancestor in the hierarchy (“MSCA”). The similarity of two cases can then be computed by converting the MSCAs for each shared feature to a numerical score (perhaps by assigning numerical “specificity values” to each node in the abstraction hierarchy) and by combining the specificity scores into an overall measure. This technique, or similar techniques, have been used in a number of systems, for example, PERSUADER [Sycara, 1990]. But the technique has many limitations that makes it unsuitable for use in the legal domain. Most importantly, there is no principled way to assign specificity values to the nodes of the Factor Hierarchy and to justify these values in arguments. Also, by assigning such values a priori, one precludes the possibility that the salience of abstractions depends on the context. Also, as Kolodner points out, in the MSCA approach “a problem arises in abstraction hierarchies when items are abstracted several different ways. This could result in several different MSCAs for the same two values. Some way of determining which are the right branches of the abstraction hierarchy to follow in computing MSCAs is needed” [Kolodner, 1993; p. 347.] An additional problem is that individual cases may present conflicting evidence for more abstract conclusions. CATO’s techniques for selecting focal abstractions address these limitations. They enable CATO to determine which abstractions are salient in a given context. CATO is able to focus on alternative interpretations of a differences, depending on an arguer’s viewpoint and the cases being compared. CATO’s techniques deal with a Hierarchy where factors may have parents along multiple paths in the hierarchy. And CATO’s techniques are able to deal with conflicting evidence related to abstractions. They resolve some of the conflicting evidence by taking into account the strength of links in the Factor Hierarchy. Other than that, CATO selects focal abstractions for which it can cite plausible support in an argument.