Predicative NPs and the annotation of reference chains

Predicative NPs and the annotation of reference chains Kaja Borthen Norwegian University of Science and Technology 7491 Trondheim, Norway, kaja.borthen@hf.ntnu.no Abstract In the development of machine learning sys- tems for identification of reference chains, hand-annotated corpora play a crucial role. This paper concerns the question of how predicative NPs should be annotated w.r.t. coreference in corpora for such systems. This question highlights the tension that sometimes appears in the development of corpora between linguistic considerations and the aim for perfection on the one hand and practical applications and the aim for efficiency on the other. Many current projects that seek to identify coreferential links automatically, assume an annotation strategy which instructs the annotator to mark a predicative NP as coreferential with its subject if it is part of a positive sentence. This paper argues that such a representa- tion is not linguistically plausible, and that it will fail to generate an optimal result. 1 Introduction Predicative NPs in positive sentences are marked as coreferential with their subject in many projects that develop corpora for coref- erence chains. This is exemplified by the an- notation scheme of MUC-7 (Hirschman and Chinchor, 1997), on which several annotation schemes are based. Also in more theoretical works this has been a common mark-up, at least for definite predicative NPs. For example, the proper name David Beckham and the definite phrase the second best player in the world in (1) below are regarded as coreferential and anaphor- ically related to each other by Mitkov (2002), whose intention seems to be to present the stan- dard view on this issue. (1) David Beckham was voted the second best player in the world behind Rivaldo. There are several arguments in favor of treat- ing a predicative NP and its subject as part of the same reference chain if they appear in a pos- itive sentence. A predicative NP and its subject are often used to describe the same individual in the world. As the goal for automatic identifica- tion of reference chains is primarily to identify equivalence sets, i.e. sets of nominals that refer to the same entity, this is a strong argument for assuming coreference between predicative NPs and their subject. A second argument in favor of this is that one otherwise loses important information about ref- erents in terms of descriptive content. Consider the following example: (2) Skogfoss is your parents’ farm, isn’t it? I remember that farm. In (2) above, we want to capture the fact that Skogfoss and that farm refer to the same entity. If one assumes an annotation strategy where Skogfoss and the predicative NP your parents’ farm are marked as part of the same reference chain, then the descriptive content contributed by farm is added to the reference chain, which in turn makes it highly likely that the later ex- pression that farm belongs to the same reference chain, as desired. In other words, the descrip- tive content contributed by a predicative phrase - if assumed to be coreferential with …

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