From Elementary Discourse Units to Complex Ones

Coherence relations have usually been taken to link clauses and larger units. After arguing that some phrases can be seen as discourse units, a computational account for such phrases is presented that integrates surface-based criteria with inferential ones. This approach can be generalized to treat intra-sentential cue-phrases. Since cue-phrases are not always present, referential relations between nominal expressions are additionally used to derive a text 's discourse structure. :1 I n t r o d u c t i o n It is widely acknowledged that texts are not just collections of sentences, but have a structure of their own. There has been substantial work in order to account for the different phenomena of discourse structure by applying theories of coherence relations, e.g. (Mann and Thompson, 1988; Asher, 1993) among others. Coherence relations represent rich semantic linkage (like Cause t or Evaluation) between text segments of varying size. However, what the minimmn size of text segments to relate should be, is still left open to debate. As common approaches argue that coherence relations relate events or situations (e.g. (Hobbs et al., 1993; Asher, 1993)) and that such events are usually introduced 1Coherence relations in this paper are basically tulc~n from taken Rhetorical Structure Theory (Mann and Thompson, 1988) will appear emphasized and Capitalized. through the means of verbs, it has become standard practice to consider clauses to be the appropriate size for elementary discourse units. It has, however, also been observed (Vander Linden and Martin, 1995; Grote et al., 1997) that sometimes phrases may serve as very condensed forms to express elaborate contents. Recently, (Schauer and Hahn, 2000) provided a more detailed analysis when prepositional phrases (PPs) may serve as elementary discourse units. Cursorily viewed, the claims of another recent study stand in contrast to the idea of intra-clansal discou~e units: (Schauer, 2000) examined the interplay of coreferential expressions and discourse structure and concluded that referential relations are a good indicator of the discourse structural configurations in case the units examiued are entire sentences. This poses the question whether not entire sentences are the appropriate grain size for elementary discourse units. I will argue that these results i.e. the different levels of granularity for discourse units are not incompatible with each other. The approach used in (Schauer and Hahn, 2000) to derive the coherence relation governing a prepositional phrase neatly carries over to the computation of coherence relations signaled by sentence-internal cue-phrases. This then allows an integration with the algorithm using referential relations that was proposed in (Schauer, 2000). 2 A d j u n c t s a s D i s c o u r s e U n i t s The question at what size of textual expressions one should start looking for discourse units has not been suftlciently answered yet.