The Local Organization of Text

In this paper, I present a model of the local organization of extended text. I show that texts with weak rhetorical structure and strong domain structure, such as descriptions of houses, digital circuits, and families, are best analyzed in terms of local domain structure, and argue that global structures that may be inferred from a domain are not always appropriate for constructing descriptions in the domain. I present a system I am implementing that uses short-raTtge strategies to organize text, and show how part of a description is organized by these strategies. I also briefly discuss a model of incremental text generation that dovetails with the model of local organization presented here. Motivat ion for local organization The approach to organizing extended text described here has both psychological and computational motivation. It aims both to model how people use language and to provide a flexible architecture for a system's language use. In this section, I describe the empirical data that form the basis of this research, and characterize the local organization of the collected texts. In the next two sections, I describe a computational architecture to implement local text organization and discuss its advantages of generality and flexibility, and give an example of how this architecture works. An extended text has a structure; this structure is a description of how the components relate so that sense can be made of the whole. Two sources of this organization are rhetoricial structure, which describes the way elements of the text fit together, and domaitt structure, which describes relations among domain objects. For this research I chose three domains with strong domain structure, and a task--descr ipt ion--with weak rhetorical structure. I have tape-recorded 29 people giving descriptions of house layouts, electronic circuit layouts, and family relationships. Description fragments of a house and of a family, and the questions asked to obtain the descriptions, are given in figure 1. (Because of space considerations, the fragments are somewhat abbreviated.) Many approaches to text organization 1 are based on analyses of text in terms of rhetorical structure. However, there are few segments of text with interesting rhetorical structure in my corpus. For example, an analysis of the texts using Mann and Thompson's (1987) Rhetorical Structure Theory (RST) would result primarily in the relations s e q u e n c e and j o i n t and would contain few of the the relations like evid e n c e or j u s t i f y that give RST its descriptive power. Similarly, it is unclear what work a system like that of Grosz and Sidner (1986) would do in analyzing a description. Since the structure of descriptions cannot be analyzed adequately with rhetorical relations, perhaps it can be explained in terms of the domain. Houses, chips, and families are strongly structured. A family's relationships can be captured in a family tree; one might suppose that a description of the family would also be organized in this way. A house can be encoded in a number of ways; for instance, it has a component hierarchy, being composed of rooms composed of furnishings. Linde (1974) has proposed another comprehensive structure for houses: a phrase structure grammar that determines how the rooms may be visited in a traversal of a house layout. Surprisingly, these global, hierarchical domain structures are not exploited in the organization of descriptions in my corpus. While family trees and composition hierarchies can be inferred from descriptions of families and houses, this does not mean that these structures guide the process of organizing them. For instance, my family informants did not simply construct their descriptions by starting at the root of the appropriate tree and doing a depth-first or breadth-first traversal of it. Instead, to select a next family member to talk about, they would apply one of several criteria. Generally, a sibling, spouse, parent, or child would be the next choice, and this might incidentally constitute part of a tree walk. But that this choice is local is evidenced by the next choice, which may not be construable, in any XWhat I call text organization is usually referred to as text planning.