The Structure of User-Adviser Dialogues: Is there Method in their Madness? 
Raymonde Guindon 
Microeleetronies and Computer Technology Corporation - MCC 
Paul Sladky 
University of Texas, Austin 8J MCC 
Hans Brunner Joyee Conner 
Honeywell - Computer Sciences Center MCC 
ABSTRACT FOCUSING AND ANAPHORA RESOLUTION 
Novice users engaged in task-oriented dialogues with an 
adviser to learn how to use an unfamiliar statistical 
package. The users', task was analyzed and a task 
structure was derived. The task structure was used to 
segment the dialogue into subdialogues associated with 
the subtasks of the overall task. The representation of 
the dialogue structure into a hierarchy of subdialogues, 
partly corresponding to the task structure, was 
validated by three converging analyses. First, the 
distribution of non-pronominal noun phrases and the 
distribution of pronominal noun phrases exhibited a 
pattern consistent with the derived dialogue structure. 
Non-pronominal noun phrases occurred more frequently 
at the beginning of subdialogues than later, as can be 
expected since one of their functions is to indicate topic 
shifts. On the other hand, pronominal noun phrases 
occurred less frequently in the first sentence of the 
subdialogues than in the following sentences of the 
subdialogues, as can be expected since they are used to 
indicate topic continuity. Second, the distributions of 
the antecedents of pronominal noun phrases and of 
non-pronominal noun phrases showed a pattern 
consistent with the derived dialogue structure. FinMly, 
distinctive clue words and phrases were found reliably 
at the boundaries of subdialogues with different 
functions. 
INTRODUCTION 
The goal of this paper is to find evidence for the notion of 
dialogue structure as it has been developed in computational 
linguistics (Grosz, 1977; Sidner and Grosz, 1985). The role of two 
hypothesized determinants of discourse structure will be examined: 
i) the structure of the task that the user is trying to accomplish 
and the user's goals and plans arising from the task; 2) the 
strategies available to the user when the user is unable to achieve 
the task or parts of the task (i.e., meta-plans). The study of 
dialogue structures is important because computationally complex 
phenomena such as anaphora resolution have been theoretically 
linked to the task and dialogue structures. 
Dialogue Structure: A Key to Computing Focus 
Given the computational expense of searching, of inferential 
processing, and of semantic consistency checking required to 
resolve anaphors, restricting the search a priori to a likely set of 
antecedents seems advantageous. The a priori restriction on the 
set of potential antecedents for anaphora resolution has been called 
focusing (Grosz, 1977; Guindon, 1985; Reichman, 1981; Sidner, 
1983). Grosz defines a focus space as that subset of the 
participant's total knowledge that is in the focus of attention and 
that is relevant to process a discourse segment. 
Task-oriented dialogues are dialogues between conversants 
whose goals are to accomplish some specific tasks by exchanging 
information through the dialogues. Task-oriented dialogues are 
believed to exhibit a structure corresponding to the structure of 
the task being performed. The entire dialogue is segmented into 
subordinated subdialogues in a manner parallel to the 
segmentation of the whole task into subordinated subtasks. Grosz 
(1977) assumes that the task hierarchy imposes a hierarchy on the 
subdialogue segments. As a subtask of the task is performed (and 
its corresponding subdialogue is expressed), the different objects 
and actions associated with this subtask come into focus. As this 
subtask is completed (and its corresponding subdialogue), its 
associated objects and actions leave focus. The task of which the 
completed subtask is a part then returns in focus. The 
segmentation of a dialogue into interrelated subdialogues is 
associated with shifts in focus occurring during the dialogue. 
Detailed task structures for each problem given in this study can 
be found in Guindon, Sladky, Brunner, and Conner (1986). 
A cognitive model of anaphora resolution and focusing is 
provided in Guindon (1985) and Kintsch and van Dijk (1978). 
Human memory is divided into a short-term memory and a long- 
term memory. Short-term memory is divided into a cache and a 
buffer. The cache contains items from previous sentences and the 
buffer holds the incoming sentence. Short-term memory can only 
contain a small number of text items and its retrieval time is fast. 
Long-term memory can contain a very large number of text items 
but its retrieval time is slow. During the integration of a new 
sentence, the T most important and R most recent items in short- 
term memory are held over in the cache. Items in focus are the 
items in the cache and are more rapidly retrieved. Items not in 
focus are items in long-term memory and are more slowly 
retrieved. Because the cache contains important items that are 
not necessarily recent, pronouns can be used to refer to items that 
have been mentioned many sentences back. An empirical study 
demonstrates the cognitive basis for focusing, topic shifts, the use 
of pronominal noun phrases to refer to antecedents in focus, and 
the use of non-pronominal noun phrases to refer to antecedents not 
in focus. 
224 
Gross and Sidner (1985) distinguishes three structures in a 
discourse structure: 1) the structure of the sequence of utterances, 
2) the structure of the intentions conveyed, and 3) the attentional 
state. Distinguishing these three structures gives a better account 
of discourse phenomena such as boundary markers, anaphors, and 
interruptions. This paper will cover mainly the second structure 
and will attempt to find evidence linking the dialogue structure to 
the task structure. The main point is that the structure of the 
intentions conveyed in the discourse should mirror to some extent 
the task structure (but see the next section). The first structure of 
the dialogue, the structure of the sequence of utterances, will 
actually be examined with the pronominal and non-pronominal 
noun phrase distributions, the antecedent distribution, and the 
boundary marker analyses. We expect that these three analyses 
will support the derived dialogue structure, the intentional 
structure. The last structure, the attentional structure, is not 
discussed here but has been discussed in Guindon (1985). ',\ 
The main point of "focusing" theories of anaphora resolution 
is that the discourse structure, based on the task structure, is a 
crucial determinant of which discourse entities are held in focus 
and are readily accessible for anaphora resolution. Subdialogues 
that are in focus are contexts that are used to restrict the search 
for antecedents of anaphors. 
Task Structure Can Only Partially Determine 
Dialogue Structure 
In any case, the task structure can only partially determine 
the goals and plans of the novice user and, indirectly, the dialogue 
structure. This is because the novice user does not have a good 
model of the task and is in the process of building one and because 
the adviser only has a partially correct model of what the novice 
user knows about the task. The verbal interaction between the 
user and the adviser is not just one of execution of plans and 
recognition of plans but rather one of situated actions and 
detection and repair of imperfect understanding (Suchman, 
1985). 
As a consequence, the dialogue structures from our data 
contained subdialogues that functioned as clarification (i.e., 
request of information) to correct imperfect understanding or as 
acknowledgement to verify understanding between the 
participants. The notion of meta-plans allows us to account for 
the presence of clarification and acknowledgement subdialogues 
(see Litman and Allen, 1984). 
RESEARCH GOALS 
There are many unanswered questions about the nature of 
dialogue structures, about the validity and usefulness of the 
concept of a dialogue structure, about the role of the task 
structure in determining dialogue structure, and in the 
contribution of the task structure to focusing and anaphora 
resolution. For example, the precise mechanisms to determine the 
initial focus and to update it on the basis of the dialogue structure 
are still unknown (Sidner, 1983). 
The goal of this paper is to find evidence for the validity of 
the notion of discourse structure derived from the task structure 
by: 1) describing a technique to derive the structure of dialogues 
and 2) validating the derived dialogue structure by three 
independent converging analyses: a) the distribution of non- 
pronominal and pronominal noun phrases b) the distribution of 
antecedents of pronominal and non-pronominal anaphors, and c) 
the presence of subdialogue boundary markers, 
If complete subdialogucs get into and out of focus and if 
subdialogues are conceived as contexts restricting the set of 
antecedents to be searched and tested during anaphora resolution, 
identifying the appropriate unit of discourse corresponding to 
these subdialogues is crucial. 
One phenomenon that should have correspondence to the 
dialogue structure is the distribution of non-pronominal and 
pronominal noun phrases. Non-pronominal noun phrases can be 
used to introduce new entities in the dialogue or to reinstate into 
focus a previous dialogue entity out of focus. In other words, non- 
pronominal noun phrases are used to indicate topic shifts. As a 
consequence, they should tend to occur more frequently at the 
beginning of the subdialogues than later in the subdialogues. On 
the other hand, pronominal noun phrases are used to refer to 
entities currently in focus. In other words, pronominal noun 
phrases are used to indicate topic continuity. As a consequence, 
they should tend to occur less frequently in the first sentence of a 
subdialogue but more frequently in subsequent sentences. 
Empirical evidence for these claims are presented in Guindon 
(1985). She found that anaphora resolution time is faster for 
pronominal noun phrases whose antecedents are in focus than for 
those whose antecedents are not in focus. On the other hand, she 
found faster anaphora resolution time for non-pronominal noun 
phrases whose antecedents were not in focus than for those whose 
antecedents were in focus. In other words, the form of the 
anaphor signals whether the antecedent is in focus (as when the 
anaphor is pronominal) or not in focus (as when the anaphor is 
non-pronominal). Grosz, Joshi, and Weinstein (1983) have made 
similar claims about the role of non-pronominal definite noun 
phrases and pronominal definite noun phrases. 
In linguistics, Clancey (cited in Fox, 1985) found that the use 
of definite non-pronominM noun phrases was associated with 
episode boundaries. Psychological evidence has shown the special 
status in memory for certain sentences in discourse found at the 
beginning of paragraphs. Sentences which belong to the 
macrostructure (i.e. gist) of the discourse have been shown to be 
recognized with more accuracy and faster than sentences belonging 
to the microstructnre (Guindon and Kintsch, 1984). 
Macrostructure sentences are by definition more abstract and 
important than microstructure sentences. They express a 
summary of the or part of the discourse. The macrostructure 
sentences tend to be the first sentences in paragraphs and be 
composed of non-pronominal definite noun phrases (van Dijk and 
Kinstch, 1983). 
Linde (1979) observed the distribution of it and that in 
descriptions of houses or apartments. She found that shifts in 
focus were associated with change in the room described. The 
pronoun it was used to describe objects in focus either associated 
with the room then described or to the entire apartment even 
when the apartment itself had not been mentioned for many 
sentences. The pronoun that was used either to refer to an object 
outside the focus or to an object in focus when the description 
of the object was in contrast with another description. Grosz 
(1977) observed a similar use of the pronoun it in her dialogues to 
the use of it in Linde's dialogues. 
225 
In summary, the most important sentences, often at the 
beginning of new paragraphs, tend to be composed of full definite 
noun phrases. These sentences often introduce a new discourse 
entity or reinstate a former one which was out of focus, creating a 
topic shift. Sentences which are nsubordinatedh to the most 
important sentence in the paragraph tend to be composed of 
pronouns and signal topic continuity. 
Another clue to dialogue structures is the distribution of 
antecedents of anaphors. Given that pronominals are used to refer 
to important or recent concepts (Guindon, 1985), the distribution 
of antecedents of pronominal anaphors should cluster in the 
current subdialogue (i.e. recency or importance), its parent (i.e. 
importance and recency), and the root subdialogue (i.e. 
importance). On the other hand, because non-pronominal 
anaphors are more informative than pronominal anaphors they 
may refer to antecedents that are more widespread in the dialogue, 
that is, antecedents that are not as recent or as important. 
Another obvious clue is the presence of reliable boundary 
markers for different subdialogue types. Some of these markers 
have been reported by Grosz (1977), Reichman (1981), and Polanyi 
and Scha (1983). The boundary markers found in our 
subdialogues should agree with those found in these previous 
analyses and extend them. 
Derivation of a dialogue structure on the basis 
of the task structure 
An important prerequisite in the interpretation of user-adviser 
dialogues is to analyze the task the users are trying to perform. A 
task analysis is a detailed description of the determinants of the 
user's behaviors arising from the task context. The first step in 
performing task analysis is to identify the objects involved in the 
task. In our case, these objects are vectors, matrices, rows, 
columns, variables, variable labels, etc. The second step is to 
identify all the operators in the task which when applied to one or 
more objects changes the. state of the completion of the task. In 
our case, these operators are function calls (e.g. mean, variance, 
sort), subsetting values from vectors, listing of values, etc. Of 
course, not every operator applies to every object. A third step is 
to identify the sequence of operators which would produce a 
desired state (the goal - e.g. the problem solved) from an initial 
state. Such a task analysis can be performed at many levels of 
abstraction, from high-level conceptual operators to low-level 
physical operators. The desired level of abstraction depends upon 
the level of abstraction of the behaviors that one wants to account 
for. Usually, the more complex or cognitive the task modelled, the 
more abstract or coarse the operators selected. In such case, the 
operators will reflect the specifics of the task environment, such as, 
vectors, matrices, screen, keyboard. The finer the grain of 
analysis, the more the operators are associated with basic motor, 
perceptual, or cognitive mechanisms. Since the task we are trying 
to model is quite cognitive in nature - solving statistical problems 
with an unfamiliar statistical package - an appropriate level of 
analysis seems to be at the level of the so-called GOMS model 
(Card, Moran, and Newell, 1983). GOMS stands for: (1) a set of 
Coals; 2) a set of Operators; 3) a set of Methods for achieving the 
goals; 4) a set of Selection rules for choosing among competing 
methods for goals. 
In the notation used in our examples, we have used a slightly 
different terminology and have used the term action instead of 
operator and use the term plan instead of method. We have also 
used the terms prerequisites, constraints, and meta-plans from 
artificial intelligence. The notion of meta-plans allowed us to 
account for the presence of clarification and acknowledgement 
subdialogues (see Litman and Alien, 1984) that could not be 
accounted directly by the task structure. 
We will now describe how the task structure was used in 
deriving the dialogue structure. Goal or plan subordination arises 
from the plan decomposition into subplans or from unsatisfied 
prerequisites. In a task structure, plans are composed of other 
plans themselves, leading to a hierarchical structure. In other 
words, a subgoal to a goal can arise from a plan decomposition 
into subplans or from the prerequisite conditions which must hold 
true before applying the plan. Here are the coding decisions used 
in deriving the dialogue structure: 
• If the user initiated a subdialogue consisting of the 
statement of a plan or of a goal, the subdialogue would 
be "inserted" in the task structure at the location of 
the plan described. 
• If the user initiated a subdialogue consisting of the 
statement of a subplan within the decomposition of its 
parent plan, the subdialogue would be "inserted" in 
the appropriate daughter subplan of the parent plan in 
the task structure. 
• If the user initiated a subdialogue consisting of a 
subplan arising from an unsatisfied prerequisite of a 
plan, then the subdialogue would be "inserted" as a 
daughter of the subdialogue associated with the plan. 
Clarification subdialogues arise from the restrictions on the 
meta-plans that the participants can use when they cannot 
achieve one of their plans: In our study, they must ask help to the 
adviser aloud. The meta-plan, ASK-ADVISER-HELP, itself has 
prerequisites, one of them being that the linguistic communication 
be successful. This leads to the linguistic clarification subdialogues 
that occur when there are ambiguities in the message that need to 
be resolved by requesting disambiguating information from the 
adviser. Another consequence of the meta-plan ASK-ADVISER- 
HELP is the presence of acknowledgement subdlalogues 
whereby participants ensure that the communication is successful 
by acknowledging that they have understood the message. 
Let's continue describing the coding scheme: 
• The clarification subdialogues are subordinated to the 
subdialogue mentioning the concept for which 
clarification is requested (e.g., goal, plan, term). 
• The acknowledgement subdialogues are subordinated to 
the subdialogue mentioning the acknowledged concept. 
• The linguistic clarification subdialogues are also 
subordinated to the subdialogue containing the 
utterance for which clarification is requested. 
• Since we are not fully modeling the user's task, 
subdialogues regarding the participants' behaviors as a 
subject in a study were ignored. 
226 
• Since knowing the required statistical formula and 
knowing how to use the console were required to solve 
all the problems, these prerequisites were not always 
encoded explicitly in the task structure. Nevertheless, 
the clarification and acknowledgement subdialogues 
regarding statistics and the use of the console were 
subordinated to the subdialogue associated with the 
plan for which these clarifications were necessary to 
obtain. 
DATA COLLECTION 
Overview of Data Collection Method 
Three novice users had basic knowledge of statistics. They 
had to use an unfamiliar statistical package to solve five simple 
descriptive statistics problems. There were two main restrictions 
imposed on the strategies employed to solve the problems: 1) the 
only source of information was the adviser; 2) all requests for 
information had to be said aloud. These restrictions were 
considered as restrictions on the mcta-plans available to the 
participants when unable to solve the problems. The participant, 
the adviser sitting to his/her right, and the console were 
videotaped. 
Coding of the Dialogues 
Each subdialogue was segmented into subdialogues which 
appeared to be the execution of a plan to satisfy a goal of the user 
or the adviser on the basis of the task structure. 
In addition to segmenting the dialogue into subdialogues, the 
relations between subdialogues were determined. One source of 
such relations is the decomposition of a total task into subtasks to 
be performed in some order. This decomposition is called the task 
structure (see Grosz, 1977) as described previously. Two 
important relations are subordination and enablement. 
Consider a dialogue occurring while performing a task, such as 
baking a cake, composed of three subtasks, (1) measure 
ingredients, (2) mix ingredients, (3) put the mixed ingredients in 
the oven. Subtasks 1, 2, and 3 are said to be subordinated to 
the task of baking a cake. Moreover, subtask 2 must precede 
subtask 3. Subtask 2 is said to enable subtask 3. The 
subdialoguss that would be instrumental to the execution of these 
subtasks would stand in the same relations. 
However, the decomposition of the task structure was not the 
only source of subordination and enablement relations between 
subdialogues. Clarification and acknowledgement subdialogucs 
even though they did not correspond to a subtask in the task 
structure were subordinated to the subdialogue introducing the 
clarified and acknowledged concept respectively. 
The coder then analyzed the distribution of non-pronominal 
noun phrases and pronominal noun phrases throughout the 
dialogue. The coder also noted words and phrases occurring at the 
boundaries of the subdialogues and mapped the distribution of the 
antecedents of pronominal and non-pronominal anaphors. 
ANALYSIS OF THE DIALOGUES 
ANALYSIS OF THE USERS' TASK 
Three main types of subdialogues have been encountered 
associated with each aspect of the task described above : 
1. Plan-goal statement subdlalogues occur when the 
user describes a goal, or a plan, or the execution of 
actions composing the plan This type of subdialogue 
may be an adjunct to the goal or plan because 
expressing them verbally might not be essential for 
their satisfaction or realization (though expressing 
them verbally helps the adviser understand the user). 
2. Clarification subdialogues occur when the user 
requests information from the adviser so that the user 
can satisfy a goal. In this study, these subdialogues 
arise from the constraints on the type of meta-plans 
available, ASK-ADVISER-HELP. There are two main 
types of clarification subdialogues: 1) those concerning 
the determination of goals and plans of the user 
(e.g., "What should I do next?", "How do I access a 
vector?"); 2) those concerning the arguments (or 
objects) in goals and plans (e.g., "What is a 
vector?"). In some cases, the clarification subdialogues 
arise from the prerequisite on the recta-plan, that is, 
assure mutual understanding. For example, the user 
will verify that he/she has identified the correct 
referent for an anaphor in the adviser's utterances. 
3. Acknowledgement subdialogues occur when the 
user informs the adviser that he/she believes that 
he/she has understood an explanation. They arise from 
the prerequisite on the recta-plan, that is, assure 
mutual understanding. 
A small subset of the graphical representation of a simplified 
subtask structure and of dialogue segmentation and structure is 
given in Figure 1 to show how the task structure partially 
influences the dialogue structure. 
\[TSZ S I DaLOGUE s'rRuCrU  
ACC~l "~aff' 
i ;. Z,:'L~., 
gXPERT: AUTO ~ a m~. ~ ly ~ "~'. 
/ 
CLAm~tCATLON. CS l~ ~ kcy~ea) 
How do I ~ntcf ~ ~w~ 
Figure 1: TASK AND DIALOGUE STRUCTURES 
227 
DISTRIBUTION OF NON-PRONOMINAL 
AND PRONOMINAL NOUN PHRASES 
Non-pronominal noun phrases play a role in indicating and 
realizing topic shifts in a dialogue. Since new subdialogues are 
assumed to correspond to topic shifts, one can predict that non- 
pronominal noun phrases will tend to occur more frequently at the 
beginning of subdialogues than later in the subdialogues. On the 
other hand, pronominal noun phrases play a role in indicating and 
realizing topic continuity in a dialogue. Since new topics are 
introduced at the beginning of new subdialogues and developed in 
the following sentences, one can predict that pronominal noun 
phrases will tend to occur more frequently after the first sentence 
in the subdialogues. As can be seen in Table 1, there is a clear 
trend for the number of non-pronominal noun phrases to decrease 
as the subdialogue progresses, especially for the most frequent 
subdialoguc lengths (i.e., 2 and 3 sentences), but less marked for 
the most infrequent subdialogue lengths (i.e., 4 and 5 sentences). 
Moreover, there is a clear increase in the number of pronominal 
noun phrases from the first sentence to the second sentence in the 
subdialogues, though again less reliable for the least frequent 
subdialgue lengths (i.e., 4 and 5 sentences). A complete statistical 
analysis of these data is presented in Guindon, Sladky, Brunner, 
and Conner (1986). 
Table 1: DISTRIBUTION OF NOUN PHRASES 
NON-PRONOMINALNOUN PHP.~ 
n~r SUBDL~GA)GUELENGTB ~ $ENTF~C~ 
2 3 4 5 
$1 234 99 30 28 
$2 114 76 49 21 
$3 46 30 22 
$4 29 20 
$5 11 
PRONOMINAl, NOUN PHRASE~ 
S¢m¢~ SUBDIALOGUE LENGTH IN SF.IqI~NCES 
~mbcr 2 
3 4 5, 
S1 13 2 5 0 
$2 24 15 4 5 
$3 9 11 2 
$4 6 4 
$5 8 
The observed distributions o£ non-pronominal and pronominal 
noun phrases follow the predictions arising from previous work in 
linguistics and psychology. Because this analysis was performed 
independently of the dialogue segmentation and subordination, it 
is a converging analysis and it supports the derived dialogue 
structure on the basis of the task structure and the users' and 
adviser's plans and goals. This analysis supports the value of the 
concept of a dialogue structure and also support our proposed 
scheme to derive such dialogue structures. 
DISTRIBUTION OF THE ANTECEDENTS 
OF ANAPHORS 
The subdialogues were indexed as shown in Table 2. The 
current subdialogue, labelled N, is the location of the anaphor to 
be resolved. All subdiMogues are indexed relative to the current 
subdialogue N. Thus, the node N-1 immediately dominates N, the 
node N-2 dominates N-I, and so on. The nodes subordinate to 
each of the nodes dominating N are indexed beginning with the 
left-most node and proceeding rightward. Thus, if N-1 is the first 
node dominating N, the left-most node subordinate to N-1 will be 
N-l/L1 and each sibling to the right will be N-l/L2, N-l/L3, etc. 
N-3 N-3 N-2 
L1 L2 -2 N~- - 
N-3 N-3 N 1 
(L1)I (L1)2, //~ L2 ~~ 
N-3 N-2 N-2 N-1 N-1 
((Li)l)l (L1)I (L1)2 L1 L2 
Table 2: INDEXING OF THE SUBDIALOGUES 
Anaphoric - Pronominal Noun Phrases 
Pronominal anaphors are used to refer to discourse entities 
that are in focus. Such entities should be either recent or of 
primary importance in the dialogue, Figure 2 represents 
graphically the distribution of the antecedents of pronominal noun 
phrases with a band, with highest frequencies shown with the 
widest bands. For sake of brevity, the exact frequencies are not 
reported here but can be found in Guindon, Sladky, Brunner, and 
Conner (1986). 
Figure z shows that the majority of pronominal antecedents 
are located in the current subdialogue, with their frequency 
decreasing as distance from the anaphor increases. The current 
subdialogue contains recent antecedents. Then, they are most 
frequently found in the parent subdialogue which contains 
important and recent antecedents. Finally, a few pronominal 
anaphors (i.e. it) have their antecedent (i.e., the statistical 
package) found in the root subdialogue which contains important 
antecedents. Grosz (1977) also observed the use of it to refer to an 
important concept that had not been mentioned for many 
sentences. These data demonstrate the existence of constraints at 
the dialogue level on the distribution of the antecedents of 
ANTECEDENT DISTRIBUTION 
Frequent 
Unfr~quen! 
= 
\[--'1 A subdkdogu¢ ¢ 
PrunominM Noun Phrase Non-prtmominal Noun Phrase 
Figure 2: ANTECEDENT DISTRIBUTION 
pronominal anaphors: most antecedents are located in the current 
subdialogue or in its immediate superordinate and a few 
antecedents co-specifying the main topic(s) of the dialogue are 
located at the root of the dialogue. 
228 
These data strongly suggest that recency plays a role within 
the current subdialogue, but also that another factor must be 
invoked to explain the high frequency of antecedents observed in 
N-1 and in the root subdialogue. This other factor is topicality 
or importance (Guindon, 1985; Kintsch and van Dijk, 1978). 
A parent subdialogue describes information that is important 
to the information described in a subordinate subdialogue. 
Moreover, the antecedent statistical package was located at the 
"root" subdiMogue of the dialogue structure. In other words, it 
was one of the most important concepts mentioned in the dialogue 
and because of its importance stayed in the user's and adviser's 
short-term memory during the complete dialogue and could be 
referred to by using a pronoun. The allocation of short-term 
memory during discourse comprehension corresponds to the 
concept of attentional state (Grosz and Sidner, 1985) and is 
described in more detail in Guindon (1985). 
The task structure and the user's meta-plans correspond to the 
intentional structure described by Grosz and Sidner (1985). Note 
that the segmentation of the task into subtasks direct the 
segmentation of the dialogue into subdialogues and is also a 
determinant of focus shifts and the attentional state. The 
antecedent distribution for pronominal anaphors is consistent with 
the dialogue structure derived from the user's plans and goals and 
describe principled and psychologically valid constraints on the use 
of pronominal anaphors over an extended dialogue. As a 
consequence, the validity of the derived dialogue structure is 
increased. 
Anaphoric - Non-pronominal Definite Noun 
Phrases 
Selecting the proper antecedent for a non-pronominal definite 
noun phrase anaphor is less difficult than for pronominal anaphor 
since more semantic information is provided for matching the 
description of the antecedent. For this reason we would expect the 
distribution for antecedents of non-pronominal definite noun 
phrases to be far less constrained than the distribution for 
pronominal noun phrases. Figure 2 shows that this is the case. 
Definite noun phrase antecedents range over every dominant node 
N-1 through N-5 and over a few left-branching subordinate nodes. 
Nevertheless, there is a strong tendency for antecedents to be 
locally positioned in N and N-1. Their distribution is consistent 
with the derived dialogue structure on the basis of an analysis of 
the task and an analysis of the users' and adviser's plans and 
goals. 
BOUNDARY MARKERS 
The analysis of boundary markers revealed reliable indicators 
at the opening of subdialogues in adviser-user dialogues. This is 
shown in Table 3. The determined boundary markers were 
consistent with those found by Grosz (1977), Reichman (1981), and 
Polanyi and Scha (1983). The boundary markers can help identify 
three major types of subdialogues: I) plan-goal statement; 2) 
clarification; 3) acknowledgement. Acknowledgement subdialogues 
occur very frequently at the end of clarification subdialogues, also 
acting as closing boundary markers for clarification subdialogues. 
A more detailed analysis of the boundary markers is given in 
Guindon, Sladky, Brunner, and Conner (1986). 
A small subset of these markers for each type of discourse act 
is given in Table 3 (the symbol ~ > means optional, "or" is 
indicated as \[ ( ) ( ) I, and ACTION means an instance from a 
class of actions). 
Subdialogue Types Boundary Markers 
................................................................... 
\[Plan-goal statement\] 1 .... <so>... I (want)(need)(have to) 
(am going to) (should) ... 
2 .... let's \[(try) (do)\[... ACTION .... 
3 .... I will ACTION .... 
Clarification 1. all types of interrogatives (e.g. How 
do I compute ..? What is a vector?) 
2. negatives expressing lack of knowledge 
(e.g .... I do not know...; ...I do not 
remember ...; ...I am not sure...) 
3. declaratives expressing uncertainty 
(e.g .... I assume that ...; ...it might be 
that ...) 
Acknowledgement 1. discourse particles (e.g. OK, Allright; 
Good) 
2 .... I \[(see)(understand)\[ .... 
3. repetition, restatement or elaboration 
of last adviser's utterance with clue 
words (e.g. In other words, ...; For 
instance ...) 
Table 2: EXAMPLES OF BOUNDARY MARKERS 
The boundary markers are part of the linguistic structure of 
dialogue, and so is the distribution of the non-pronominal and 
pronominal noun phrases. Both analyses are consistent with the 
derived dialogue structure on the basis of the task structure and 
the users' and adviser's plans and goals and they increase the 
validity of the derived dialogue structure. Both analyses also show 
that shifts in focus during discourse comprehension can be 
signalled in the surface form of the conversants' utterances. As a 
consequence, they can be capitalized upon by natural language 
interfaces. 
CONCLUSION 
Three independent converging analyses support the dialogue 
structure derived on the basis of the task structure and the users' 
and adviser's plans and goals. The distribution of the non- 
pronominal noun phrases shows that they occur more frequently at 
the beginning of subdialogues than later in the subdialogues, as 
should be expected if non-pronominal noun phrases introduce new 
entities in the dialogue or reinstate previous ones. The 
distribution of the pronominal noun phrases show that they occur 
less frequently in the first sentence than in the second sentence of 
the dialogue, as can be expected if they act as indicator of topic 
continuity. The distribution of pronominal antecedents shows that 
speakers are sensitive to the organization of a dialogue into a 
hierarchical structure composed of goal-oriented subdialogues. 
Antecedents of pronominal noun phrases tend to occur in the 
current subdialogue, in its parent, or in the root subdialogue. In 
particular, concepts mentioned in the current subdialogue, its 
parent, or in the root subdialogue tend to be in focus. In the case 
of non-pronominM definite noun phrase anaphors, while it is 
possible for antecedents to be much more widely spread across the 
dialogue, they also tend to be located in the current subdialogue or 
its parent. As a consequence, it would be possible to restrict and 
order the search for the antecedents of pronominal and non- 
pronominal definite noun phrases on the basis of the type of 
dialogue structure exemplified in this paper. The analysis of 
boundary markers reveals reliable and distinctive surface linguistic 
markers for different types of subdialogues. 
229 
The notion of a dialogue structure based on the task structure 
has been empirically supported. The notion of focusing and its 
relation to the segmentation of the dialogue into subdialogues has 
also been supported, especially by the antecedent distribution of 
the pronominal and non-pronominal noun phrases. The results of 
Guindon (1985) showing different anaphora resolution times for 
different types of anaphors with antecedent in or out of focus also 
support the refocusing" theories of anaphora resolution. This gives 
an impetus to include a model of the dialogue structure and a 
focusing mechanism in natural language interfaces. However, 
much further work has to be done to define precisely how the 
dialogue structure could be computed from the task structure and 
the meta-plans of the conversants and how precisely the anaphora 
resolution process would capitalize on this structure. 
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