RAFT/RAPR and centering: a comparison and discussion of problems related to processing complex sentences
Linda Z. Suri, Kathleen F. McCoy · 1994
abstract preferences underlying the frameworks share much in common, the approaches very often make the same predictions. RAFT/RAPR's focusing and pronoun resolution algorithms reflect the following underlying abstract preferences: We prefer for the CF to be something that is co-referential with an element mentioned earlier in the discourse. We prefer for the CF to be a pronoun rather than a full definite description. We use preferences for resolving pronouns and computing the CF that involve the grammatical role of the pronoun. We prefer for the CF to be the same as the last CF and the SF to be the same as the last SF. Centering shares the first two of these in common with us. But, centering prefers the local focus to stay the same and for the subject to be the local focus (which is stronger than preference 4). As noted earlier, we do not apply preference 2 without regard for the grammatical roles of the NPs and which focusing data structures they co-specify. Furthermore, centering invokes the second preference during pronoun resolution, while we invoke it only after resolving the pronouns. In addition, centering requires that the focus (Cb) realizes an element in the immediately preceding sentence, as opposed to merely preferring for the focus to be something in the immediately preceding sentence but allowing it instead to be co-referential with an element further back in the preceding text (as RAFT/RAPR does). In sum, the order in which these abstract preferences are applied differs between the two frameworks. Furthermore, as a result of (some) differences between the frameworks with respect to these preferences, resolvhlg a pronoun to have the same grammatical role as its antecedent is less important in centering than it is in RAFT/RAPR (Section 4.3). RAFT/RAPR presents possible referents for pronouns one possibility at a time, and if pragmatic, semantic, and general knowledge inferencing rejects a referent, RAFT/RAPR proposes an alternative. Centering, on the other hand, (in addition to requiring this same kind of inferencing) sometimes suggests multiple possibilities for the co-specifications of pronouns in a sentence (Walker 1989). Is pragmatic inferencing applied at this point to pick among the possible antecedents? We assume this to be the case, otherwise it is not clear how centering will choose among the multiple potential referents. Yet, assuming this is the case, centering seems to involve more complex inferencing than our approach involves; inferencing for centering must pick which antecedent is better. RAFT/RAPR only asks inferencing to confirm a co-specification. 19 19 We do not check for ambiguity of pronouns in the same way that Sidner did. However, even if we were to incorporate similar checks for ambiguity, such checks would involve using inferencing to confirm two possible co-specifications for a pronoun, while centering might require the use of inferencing to confirm more than two. Thus, the inferencing required by RAFT/RAPR would still be more limited than that required by centering. 315