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 blockscan 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. 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] , , 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 mutually by the participants as the discourse progresses. 276 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. 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 primary 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, bottom-up process, after which recursive organizational strategies can be applied at lower levels. 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. 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 mutually exclusive; nothing should belong to more than one. 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. 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 primary 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 testimony fi'om which the basic block in Figure 1 was extracted 4. This block references a particular time frame: zero to thirty seconds of the accident at Three Mile Island 5. The remaining blocks of that testimony are similarly constructed around time 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. What 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. Now, what happened at Three Mile Island was that a feedwater transient was experienced, and by that I mean, simply, we lost feedwater to the plant momentarily. Now, wittl loss of feedwater to the steam generator, the plant will experience a momentary pressurization above its normal pressure. This pressurization is released by a relief valve located at the top of the pressurizer. It simply opens and lets out a little bit of stean't to take care of the excess pressure that isinterruptionThen at 15 seconds into the event-keeping in mind that the valves opened maybe 5 seconds into the event-at 15 seconds the pressure started coming down because the valve had opened and cut off the pressure. The valve should have reclosed when it got back down to about 2,250 pounds; it did not reclose. The pressure proceeded to come on down. At about 30 seconds into the event, this water here started disappearing, of course, because now you are continuing to remove a very large amount of heat here, which is then coming off as secondary side steam generation, and this water will proceed to disappear if you do not start replacing it. And the auxiliary feedwater which normally comes on to make sure this does not go dry came on at about 30 seconds into the event. And at least the pumps were running. So this picture here is just the first 30 ,~econds and-or thereabouts. And this figure here is indicative of the situation from 1 to 6 minutes into the event. Figure 1 , the speaker closes the block with explicit reference to its focus: "So this picture here is just the first 30 seconds and-or thereabouts." The subsequent block ii~ this testimony is also well marked, this time by its initial sentence (which is the last sentence of the figure): "And this figure here is indicative of the situation from 1 to 6 minutes." The basic block structure is further distinguished by certain forms of repetition, whose use is closely tied to the basic block structure. While a brief allusion to these forms is made below, a detailed discussion is contained in [MCM89]. OF COHERENT, Given tile existence of a high-level structure as evidenced by explicit markings and repetition, we must consider how such a framework is chosen. If the only consideration were the identification of a set of related foci which can partition the material to be conveyed, then any set of related concepts about which the material can be cohesively organized would suffice. Consider the motivation behind the block of Figure 1 . On the surface, the events in this block all occurred within the first thirty seconds of the accident. However, it is doubtful whether the driving force behind the construction of this block was to communicate the time frame in which these events occurred; rather, what is of importance is their relative sequence in the total series of events, their causeeffect relations, and their impact on the resulting accident. One could argue that the individual events in this block represent a cause-effect chain, and hence their mutual grouping; but this chain is continued in the next block of the testimony. Thus, this argument alone cannot account for this segmentation. Apparently, other factors beyond the ability to cohesively juxtapose clauses contribute to the segmentation and the high-level framework about which it is constructed. In all of the dialogues examined, the blocks are of approximately the same size. Apparently, speakers choose organizations that tend to be well-balanced. However, balance does not seem to be the only criterion taken into consideration; if it were, we could expect to see perfectly balanced organizations in which each of the concepts to be conveyed is realized by its own basic block in addition to those well-bahmced organizations found in our text analysis whose blocks are comprised of many concepts. The size of the component basic blocks seems to be a further consideration in determining what constitutes an acceptable framework. No lengthy discourse organized about a single primary segment has been observed, nor have any in which primary segments are composed of single concepts. The "ideal" size of a basic block seems to be three or four paragraphs in length for an explanation of three pages. We contend that the high-level structure of extended explanations reflects the characteristics of an "ideal" framework in which: The basic block loci about which the material is organized reflect various aspects of the unifying framework. • Basic blocks are the same size. • The length of each basic block approximates the ideal size. Our basic block model of discourse posits that the attainment of each of these features, termed compositional goals, is instrumental in the selection of a high-level framework. However, not all observed explanations conform to the basic block model as presented thus far. For example, in explanations that are concluded by a final 278 summary, one would expect tile summary to emphasize the major points stressed by the speaker in the body of the text. In fact, there is a strong correlation between the segments comprising such a summary and the foci about which the basic blocks are constructed (see [MCM89]). Ilowever, some explanations have been examined in which material that appears in a summary has not been included in the
Another approach REF ) is bottom-up and is used to group together stretches of text in a long, generated document by finding propositions that are related by a common focus.
The Generation Of High-Level Structure For Extended Explanations
International Conference On Computational Linguistics
Computer Science