The Generation Of High-Level Structure For Extended Explanations

2 This paper analyzes the structural features of naturally-occurring extended explanations and argues that current generation methodologies are inadequate for determining high-level structure. It presents a computational model based on the hypothesis that high-level structure composed of a unifying framework and its associated basic blocks can be determined by bottom-up processes that attempt to satisfy speaker, listener, and compositional goals, after which top-down strategies can be used to organize the material about the selected framework. 1 I N T R O D U C T I O N In this paper, we describe the structural characteristics of extended, planned 1 explanations involving complex physical devices and present a computational model for generating such explanations. Our investigation suggests that the organizational strategies currently employed for structuring short explanations are inadequate for generating the high-level structure characteristic of that found in naturallyoccurring extended explanations, which typically require several pages of text. Our computational model is based on the hypothesis that text structure is not completely recursive as others have claimed ([GS86], [Rei78], [Po186], [MT88]), but rather that the highlevel structure of extended explanations is deter-. mined by processes separate from those which organize text at lower levels. Section 2.1 provides a brief overview of current models for structuring text, followed by a description of the basic block, the unit of discourse on which our model is based, in Section 2.2. Section 2.3 describes the characteristics of high-level structure of extended explanations, followed by a description of our strategy for generating this structure in Section 3; a complete description is contained in [MCM89]. 1Emphasis is placed on planned to distinguish these explanations from discourse in which the material is developed mutual ly by the part icipants as the discourse progresses. 276 T H E B A S I C B L O C K M O D E L OF E X T E N D E D D I S C O U R S E ill order to generate an extended explanation, a natural language system must determine tile basic content to be conveyed; the next step is to cohesively organize this material. As anyone who has had to organize large amounts of information into a coherent text can attest, there are many possible combinations of that material, some more cohesive than others. Frequently, deciding how to orgauize a large body of material is more difficult than determining what to include. Our research is concerned with the identification of a coherent unifying framework about which an extended explanation can be organized and the criteria for selecting from among several frameworks when more than one viable alternative exists. 2.1 O T H E R A P P R O A C H E S T E X T S T R U C T U R E TO 2.1.1 C O M P U T A T I O N A L A P P R O A C H E S A number of researchers (e.g., [GSS6], [MT88], [Rei7S], [PolS6]) have argued that discom-se is con> posed of hierarchically structured segments and that this structure is completely recursive in nature. Two general methodologies have been applied to the structuring of explanations: schemas ([McK85], [McC85], [earS7]) and rhetorical structure theory (RST) ([HMS9], [HovS8], [MPS8], [MSSS]). A schema is a discourse strategy that captures a typical pattern of discourse associated with a particular discourse purpose, (e.g., providing an analogy or evidence). Schemas can be thought of as templates composed of an ordered sequence of rhetorical predicates, which "characterize the predicating acts a speaker may use and delineate tile structural relation between propositions in a text." ~' 2From [McK85] page 9 These predicates are intended to capture the structural relations that hold between clauses in a text. The predicates are used recursively, capturing the structure of text at any level. RST, developed by Mann and Thompson ([MT88]), was originally a tool for the analysis of text. RST claims that , except for a small number of highly-stylized forms, all coherent texts have an RST ,decomposition. RST posits a small number of relations, comparable to McKeown's rhetorical predicates, that exist between segments of text. Because each relation has associated with it well-defined intended effects and conditions necessary for it to hold, RST lends itself well to a generation methodology based on a top-down, hierarchical planning formalism ([Sac77]). Thus, like MeKeown's rhetorical pred-icates, RST claims to account for the structure of text at any level of the discourse hierarchy. While these methods have proven to be effective for organizing short pieces of text, we maintain that they are inadequate for generating the characteristic structure of extended explanations at the level of the primary segments, which occupy the first level of the discourse hierarchy. We contend that the characteristics exhibited by the pr imary segments of extended explanations, to be described in the next section, cannot be captured by recursive processes. Rather, we maintain that high-level structure must be gener-ated 'by a separate, bo t tom-up process, after which recursive organizational strategies can be applied at lower levels. 2.1.2 R H E T O R I C Rhetoric, the formal study of the art of good writing, provides general strategies for organizing text at a high level that are absent from the computational models. Analysis "the method of explanation whereby a subject is divided into its separate component parts ' '3 is possibly the most instrumental of these strategies. There are no hard-andfast rules for determining what constitutes an appropriate analysis of a subject. As [WA60] observes, a subject may be classified in as many ways as it has character is t ics /par ts /s tages/e tc . However, there are three criteria which experts ([WA60], [Are75], [Tho57], [Dan67], [KP66]) mutual ly consider essential for a satisfactory organizational strategy: 1. The tent (e.g. scheme should be logical; a single, consiscriterion should be used for tile analysis time, steps in a process). 2. The scheme should exhaust all of the possibilities; everything to be conveyed should be encompassed by the scheme. 3. The resultant categories should be mutual ly exclusive; nothing should belong to more than one. 3[Are75] page 107 While the type of explanation with which this paper is concerned exhibits a high-level organization reflective of these criteria, the criteria by themselves do not provide the specificity necessary for computational generation. These guidelines include no suggestions for dealing with situations in which no logical, all-inclusive framework Call bc identified, nor do they offer suggestions for selecting among several organizational schemes which meet the prescribed criteria equally well. Furthermore, the guidelines are not sufficient in-and-of themselves to account for all of the observed phenomena discussed in the following sections. 2.2 B A S I C B L O C K S Our model is based on a discourse unit which we have termed a basic block. A basic block consists of two elements: 1. an organizational focus, such as a person or location, and 2. a set of concepts related to that focus. The focus is what makes a cohesive unit of the material in the block; it is the thread cominon to all of this material , whether directly or indirectly. A basic block will be realized as a pr imary segment of text which occupies the first level of the discourse hierarchy. In a coherent discourse, the loci on which the basic blocks are based are themselves related, each representing a different aspect of some unifying framework. These points are demonstrated by the test imony fi'om which the basic block in Figure 1 was extracted 4. This block references a particular t ime frame: zero to thirty seconds of the accident at Three Mile Island 5. The remaining blocks of that testimony are similarly constructed around t ime frames, e.g., one to six minutes, six minutes to one hour, etc. Observed frameworks demonstrate a gamut of types: properties of the concepts (location, time), planning strategies in which events are involved (medical diagnosis), and characteristics that are not only inherent in the material but also due in part to the speaker 's perception of them (significant factors). There appears to be no limit to what can constitute an acceptable framework, only that it is derived from the material itself and not from an independent device solely concerned with text structure. Wha t may be a potential framework for one set of material may be totally inadequate for another. Note that these features are reflective of the guidelines suggested by analysis. In addition to forming a cohesive unit, basic block structure is explicitly distinguished in the following two ways. First, it is often explicitly marked. In 4 Space limitations prevent inclusion of the complete text. 5Three Mile Island is a nuclear power plant located in the state of Pennsylvmfia in the United States. It suffered a nearmeltdown in 1979.