Pronominalization revisited

Pronominalization revisited∗ Renate Henschel and Hua Cheng and Massimo Poesio HCRC, University of Edinburgh, UK {henschel,huac,poesio}@cogsci.ed.ac.uk Abstract Pronominalization has been related to the idea of a local focus – a set of discourse entities in the speaker’s centre of attention, for example in Gundel et al. (1993)’s givenness hierarchy or in centering theory. Both accounts say that the determination of the focus depends on syntac- tic as well as pragmatic factors, but have not been able to pin those factors down. In this paper, we uncover the major factors which de- termine the focus set in descriptive texts. This new focus definition has been evaluated with re- spect to two corpora: museum exhibit labels, and newspaper articles. It provides an opera- tionalizable basis for pronoun production, and has been implemented as the reusable module gnome-np. The algorithm behind gnome-np is compared with the most recent pronoun gener- ation algorithm of McCoy and Strube (1999). 1 Introduction Besides the well established problem of pronoun resolution, pronoun generation is now attract- ing renewed attention. In the past, generation systems generated pronouns without attaching much importance to the problem, one notable exception being the classical algorithm of Dale (1990), loosely based on centering theory. With the emergence of corpus based studies in compu- tational linguistics, the question arises whether it is possible to refine known standard algo- rithms, or whether an improvement is only to be achieved with the help of world knowledge reasoning – a matter too complex to be dealt with reliably at this time. The former direction is represented by the pioneering work of McCoy and Strube (1999). They propose a refined algo- rithm for the choice between definite description on the one hand and pronoun on the other for ∗ The work reported in this paper has been carried out with the financial support of UK ESPRC grant L51126. animate referents1 , which is based on distance, time structure and ambiguity constraints. Here we introduce a more general algorithm for the pronominalization decision that is valid not only for animate but for inanimate referents as well. In conformity with McCoy and Strube, we group noun phrases with definite determiner and proper names together under the term “def- inite description”. The algorithm proposes a new pronominalization strategy, which beyond McCoy and Strube (1999)’s criteria makes use of the discourse status of the antecedent and parallelism effects. The algorithm has been implemented as the reusable module gnome-np. It has been re-used in the web hypertext generation system ILEX (see Oberlander et al. (1998)). It shows an accuracy over 87% with respect to two corpora (each 5000 words) of different genres. 2 Accounts of pronominalization In previous accounts pronominalization has been related to the idea of a local focus of at- tention: a set of discourse referents who/which is in the center of attention of the speaker (e.g. Sidner (1979), givenness hierarchy (Gundel et al., 1993), centering theory (Grosz et al., 1995), RAFT/RAPR (Suri, 1993)). Whereas (Gundel et al., 1993) do not …

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