An Empirical Model of
for Spoken-Language Systems
Interactive Systems Group
Department of Computer Science and Engineering
Oregon Graduate Institute
20000 N.W. Walker Rd.
P.O. Box 91000
Portland, OR 97291-1000
We refine and extend prior views
of the description, purposes, and contexts-of-use of acknowledgment acts
through empirical examination of the use of acknowledgments in task-based
conversation. We distinguish three broad classes of acknowledgments (other→ackn,
self→other→ackn, and self+ackn) and present a
catalogue of 13 patterns within these classes that account for the specific
uses of acknowledgment in the corpus.
This study is motivated by the
need for better dialogue models in spoken-language systems (SLSs). Dialogue
models contribute directly to the interaction by providing inter-utterance
coherence. Fluent understanding and use of acknowledgments should improve
spoken-language systems in at least the following ways:
- Preventing miscommunication. Acknowledgments are an important
device for establishing mutual knowledge and signaling comprehension.
Early detection and correction of cases of miscommunication and
misunderstanding should prevent failure that would otherwise have been
even more catastrophic.
- Improved naturalness. Acknowledgments are a prominent
feature of human-human dialogue. Supporting the use of acknowledgments for
both the system and the user will emphasize the “naturalness” of
interfaces and improve their utility.
- Dialogue control. Humans cope with dialogue control (e.g.,
turn-taking) with seemingly little or no effort. Acknowledgments form an
intricate relationship with dialogue control mechanisms. Understanding
these control mechanisms is central to the development and success of
spoken language systems in order to “track” dialogues and determine
appropriate system actions.
- Improved recognition. To the extent that a dialogue model
can narrow the range of possible contexts for interpretation of a user’s
utterance, a spoken-language system’s speech recognition performance will
be improved (Young et al., 1989) .
We seek to refine and extend
prior views of the description, purposes, and contexts-of-use of acknowledgment
acts through empirical examination of the use of acknowledgments in task-based
conversation. In particular, we seek to describe systematically (1) the
communicative value of an acknowledgment and (2) the circumstances of its use.
The scope of our inquiry involves spoken interaction. We present a catalogue of
types of acknowledgment. This catalogue is based on a process model of
acknowledgment that explains instances of these acts in a corpus of task-based
2 Related Work
Clark and Schaefer (1989)
suggested that acknowledgments are an important component of a larger framework
through which communicating parties provide evidence of understanding.
Conversants have a range of means, which vary with respect to strength, for
indicating acceptance of a presentation. These include continued attention,
initiation of the next relevant contribution, acknowledgment, demonstration and
Thus acknowledgments are common
linguistic devices used to provide feedback. Broadly speaking, acknowledgments
are responsive acts.1 That is, they are usually
uttered in (possibly partial) response to a production by another speaker;
acknowledgment acts express beliefs and intentions of one conversant with
respect to the mutuality of prior exchanges involving some other conversant.
The intended perlocutionary effect of an acknowledgment act is generally the perception
of mutuality of some belief.
In previous research, the
function of acknowledgments has been most readily characterized in terms of
attention, understanding and acceptance on the recipient’s behalf (Kendon,
1967; Schegloff, 1982). In addition, it has been suggested that they serve to
facilitate active participation in dialogues and promote "smooth"
conversations (Duncan & Fiske, 1987).
Schegloff (1982) described two
main types of acknowledgment: continuers and assessments. Continuers, such as “uh
huh,” “quite,” and “I see,” act as bridges between units. Conversants use
acknowledgments as continuers to signal continued attention and to display the
recipient’s understanding that the speaker is in an extended turn that is not
yet complete. Moreover, continuers indicate the turning down of an opportunity
to undertake a repair subdialogue regarding the previous utterance or to
initiate a new turn. Assessments--such as “oh wow” and “gosh, really?”"--are
essentially an elaboration on continuers. That is, they occur in much the same
environment and have similar properties to continuers, but in addition express
a brief assessment of the previous utterance.
Empirical analysis of
conversations has indicated that the occurrence of acknowledgments is not
arbitrary. Acknowledgments mostly occur at or near major grammatical
boundaries, which serve as transition-relevance places for turn-taking (Sacks et
al., 1974; Hopper, 1992). In particular, work by Orestrom (1983) and
Goodwin (1986) suggested a tendency for continuers to overlap with the primary
speaker’s contribution, in such a way that they serve as bridges between two
turn-constructional units. Assessments, on the other hand, are generally
engineered without overlap. Goodwin suggested that conversants make special efforts
to prevent assessments from intruding into subsequent units. That is, the
speaker typically delays production of the subsequent unit until the recipient’s
assessment has been brought to completion.
Clearly, acknowledgments are an
important device for providing evidence of understanding and for avoiding
miscommunication between parties. Just as next-relevant-contributions include
the entire range of potential task or domain actions, the task-based role of
acknowledgments can be differentiated within their class as acceptances. Beyond
continuers and assessments, we will demonstrate that acknowledgments
incorporate a larger set of conversational actions, many of which relate to
coherence of multi-utterance contributions.
3 Dialogue Analysis
this section, we describe the task characteristics and the corpus used for this
study, present a theoretical model of acknowledgment acts in task-based
dialogue, and present an analysis of acknowledgment acts based on corpus
3.1 The Vehicle Navigation System
corpus we analyzed was collected by U S WEST Advanced Technologies in the
domain of a vehicle navigation system (VNS). A VNS is intended to provide
travel directions to motorists by cellular telephone: the system interacts with
the caller to determine the caller’s identity, current location and
destination, and then gives driving directions a step at a time under the
caller’s control. U S WEST collected the dialogues through a Wizard-of-Oz
experiment (Brunner et al., 1992) in which the wizard was free to engage
in linguistically unconstrained interaction in the VNS task. Each of the 21
subjects performed three tasks in the VNS domain. As a whole, the corpus
contained 2499 turns and 1107 acknowledgments.
3.2 A Task-Based Model of
generally accepted view of acknowledgments, as noted earlier, distinguishes
between two classes--namely continuers and assessments (Schegloff, 1982).
Indeed, there were many occurrences of continuers (and a few assessments) in
the VNS corpus. However, our analysis suggests that acknowledgments perform
functions beyond these two classes. For instance, we observed several instances
of acknowledgment being used at the beginning of a turn by the same speaker.
This contrasts with the notions of continuers and assessments which, by
definition, occur as unitary productions in the context of extended turns by
another speaker. Clearly, an acknowledgment occurring at the beginning of a
turn is not serving as a prompt for the other speaker to continue.
account for the evidence provided by the VNS corpus, we propose to extend
Schegloff’s classification scheme into a task-based model of acknowledgment
acts. This model formalizes the meaning and usage characteristics of
acknowledgments, based on an analysis of what acknowledgments mean and when
acknowledgments are used in the VNS dialogues. A useful way of looking at the
role of acknowledgments in the context of turns is to consider the basic
structural context of exchanges. We begin by reviewing the concept of an
adjacency pair (Schegloff & Sacks, 1973; Clark & Schaefer, 1989). An
adjacency pair is formed by two consecutive utterances that have a canonical
relationship, such as question-answer and greeting-greeting. An acknowledgment
can be produced as the second phase of an adjacency pair or following a
complete adjacency pair; in each case, the utterance may contain multiple
acceptances. Of course, an acknowledgment can be produced also as a single turn
that does not relate to an adjacency pair. Thus, based on exchange structure
one can distinguish three broad structural classes of acknowledgments:2
Other→ackn, where the
acknowledgment forms the second phase of an adjacency pair;
Self→other→ackn, where Self
initiates an exchange, Other (eventually) completes the exchange, and Self then
utters an acknowledgment; and
Self+ackn, where Self includes an
acknowledgment in an utterance outside of an adjacency pair.
the other→ackn class, the exchange is a basic adjacency pair;
Other’s act will be composed of a single turn. In the self→other→ackn
class, the exchange initiated and eventually acknowledged by Self may be
composed of multiple turns, with multiple utterances by both Self and Other. In
the self+ackn class, the acknowledgment occurs in a single, extended
turn by Self that may contain multiple utterances.
3.3 A Catalogue of Acknowledgment
Acts in Task-Based Dialogue
this section, we elaborate the structural classes of acknowledgment through a
catalogue of patterns of speech acts that occur in each class. This catalogue
provides broad coverage of patterns typically encountered in task-oriented
discourse. These patterns describe the context of acknowledgments in terms of
exchanges and are derived from utterances in the VNS corpus. Each act in an
exchange is represented in terms of speech-act verbs based on the set described
by Wierzbicka (1987).Table 1 summarizes the speech-act pat- terns in the
catalogue. In the following sections, we will consider each of the structural
classes in turn and provide examples of selected patterns from the VNS corpus.
We also consider embedded exchanges, in which basic patterns are used to build
more complex patterns.
Table 1: A Summary of
Speech-Act Patterns for Structural Classes of Acknowledgment
Acknowledgments in the other→ackn class relate principally to the
immediately antecedent utterance as opposed to the prior exchange, which is
covered by the self→other→ackn class. In Clark and Schaefer’s (1989)
terms, Self’s acknowledgment in the other→ackn class serves as the
acceptance phase for Other’s presentation. As listed in Table 1, the canonical other→ackn
patterns are inform→ackn, inform→ackn+mrequest, request→ackn+inform,
mdirect→ackn and preclose→ackn.3
In each of these cases, the first turn is by Other and the second turn is Self’s
acknowledgment. In some cases, Self’s turn also extends to include other
significant utterances. We illustrate the other→ackn class through
examination of the inform→ackn and inform→ackn+mrequest
inform→ackn pattern covers cases where Other performs an
inform act and Self responds with an acknowledgment of that act. In the
following example 4 of an inform→ackn, the wizard gives directions
to the user, who acknowledges these directions. This is an example4 of
an acknowledgment that Schegloff (1982) would call a continuer.
Wizard: On Evans, you need to turn left and head West
for approximately three quarters of a mile to Clermont.
Wizard: And, um, on Clermont you turn left, heading
South for about two blocks to Iliff.
the “okay” at turn 1.2 indicates the user’s acceptance of the wizard’s
utterance. That is, the acknowledgment implies that the user understood
information given by the wizard| more emphatically than a simple next-relevant-contribution
response. Use of the acknowledgment would be strong evidence of understanding
in Clark and Schaefer’s (1989) terms. An important point to stress here is that
the wizard cannot rely on the user necessarily having received the information
that was actually conveyed or formed the intended interpretation. Rather, the
wizard is left with the user’s response indicating that the user was apparently
satisfied with the wizard’s original presentation.
The inform→ackn+mrequest class represents a significant
functional variation on the inform→ackn class just considered. It
covers cases where Other performs an inform act, Self responds with an
acknowledgment of that act and then goes on to seek clarification of the
content of the inform act. Requests for clarification are kinds of meta- act
because they are concerned with aspects of dialogue control rather than the
task itself. That is, requests for clarification are concerned with the
specifics of old information rather than seeking to elicit largely new
information|unlike request- inform acts.
Example 2 (U4.3.1)
Wizard: Okay. Then you want to go north on Speer
Boulevard for one and one half miles to Alcott Street.
User: Okay. I want to go right on Speer?
Wizard: It will be a left.
In this example, the repair is a potential re- quest for specification
(Lloyd, 1992). That is, the user’s clarification at 2.2 (“I want to go right on
Speer?”) focuses on information which was missing from the surface structure of
the original inform act but which is potentially available, namely “right”
instead of “north.”
Acknowledgments in the self→other→ackn class relate to the
previous exchange, rather than just the previous utterance. Broadly speaking,
they express the current state of the dialogue in addition to embody- ing the
functionality of other→ackn acknowledgments. That is, they
explicitly mark the completion of the antecedent exchange and indicate that the
dialogue will either enter a new exchange or resume an existing exchange.
Furthermore, self→other→ackn acknowledgments signal understanding
and acceptance of both the previous exchange and the previous utterance. The
canonical patterns of the self→other→ackn class, as listed
in Table 1, include inform→ackn →ackn, request→inform→ackn,
mrequest→inform→ackn and mdirect→ackn→ackn.
We illustrate the self→other→ackn class through examination
of the request→inform→ackn pattern.
The request→inform→ackn class covers cases where
Self requests an inform act of Other. Other then performs that inform act and
Self acknowledges. Note that the acknowledgment in this case follows a
completed request-inform adjacency pair.
Earlier, we mentioned that question-answer adjacency pairs can be
regarded as special cases of request-inform adjacency pairs (Searle, 1969). In
the following example, the wizard requests the user’s start location. The user
satisfies this request by communicating the desired information and the wizard then
acknowledges. Here the acknowledgment at 3.3 serves to indicate acceptance
(that is, receipt, understanding and agreement) of the user’s inform act and is
a signal that the request initiated by the wizard at 3.1 has been satisfied and
thus the exchange is complete.
Example 3 (U2.1.1)
Wizard: Okay anduh,what’s your starting location?
User: I’m at 36th and Sheridan at the Park-n-Ride.
Wizard: Okay, one moment please.
Self-acknowledgments occur when Self issues an acknowledgment following some
action (either verbal or physical) performed by Self. These are not responsive
acts, unlike other acknowledgment usages considered; however, they are still
closely tied with the idea of establishing mutual beliefs. The canonical patterns,
as listed in Table 1, include inform+ackn+inform, mrequest+ackn,
and mdirect+ackn. We illustrate the self+ackn class through
examination of the inform+ackn+inform pattern.
In this pattern, Self uses an acknowledgment in the middle of an
extended turn. Consider the following example:
Example 4 (U5.3.1)
(4.1) Wizard: All right, um, the _rst thing you
need to do is go South on Logan Street for one and a half miles to Evans
Avenue. Then turn left on Evans Avenue and go one and a quarter miles to South
Josephine Street. Okay, then you’ll turn left on South Josephine Street. Nineteen
Forty South Josephine is within the first block.
This particular self-acknowledgment is very similar to a
continuer|indeed it may be regarded as a self-continuer. The wizard’s
acknowledgment in this example represents a sort of temporizing, a chance for
the wizard to “catch his mental breath.” For the user, this sort of “Okay” thus
signals that the wizard intends to continue his turn. This is functionally distinct
from a meta-request of the form “Okay?” because there is no rising inton tion
and the wizard does not wait for a response. In fact, use of a self-acknowledgment
at the end of a turn would be peculiar.
Exchanges We noted earlier that basic patterns can used to build
more complex patterns. This can lead potentially to variations in patterns of
acknowledgments. In particular, it is possible to observe cascades of
acknowledgments as nested exchanges are “popped” one by one. Simple acts may be
replaced by more complex exchanges, so that an inform act may be replaced by an
exchange that accomplishes an in- form via a sequence of informs, clarifications
In this section we will consider one of the variations encountered in the
VNS corpus; where an inform→ackn→ackn replaces the inform
act in an inform→ackn sequence. In the following example, there
are three successive acknowledgment acts. The first acknowledgment at 5.2 is
accompanied by a verbatim response by the user. It is the second phase of the inform→ackn
adjacency pair, indicating understanding and acceptance of the wizard’s inform
act in which a direction was clarified. The second acknowledgment, issued by
the wizard at 5.3, marks the completion of the inform→ackn
exchange. That is, the wiz- ard recognizes that it is his or her turn yet has
nothing more to add, so indicates passing up the turn with an acknowledgment.
The third acknowledgment, issued by the user at 5.4, is associated with the
user recognizing that the wizard has finished clarifying directions; the user
thus acknowledges this embedded inform act. The user then indicates
satisfaction and approval of the wizard’s directions with the assessment “Sounds
Example 5 (U6.2.1)
Wizard: Okay, it was, um, on Evans it’s three and three
quarter miles to Jasmine.
User: Three, okay.
User: All right, sounds good.
Why is a conversation-analytic study of acknowledgment useful in the
development of spoken language systems? SLSs developers face the dual
challenges of creating both domain-based dialogues and repair-oriented
dialogues. Lacking systematic mechanisms for natural maintenance of mutuality,
SLSs tend to rely on domain structures, producing rather stolid interaction.
The most advanced systems incorporate repair acts, but are unable to relate the
repairs to the main dialogue structures in a natural way. The acknowledgment
model described in this paper provides a systematic method of maintaining mutuality
of knowledge for both domain and control information. More concretely, using
this model SLSs can account for acknowledgments by both user and system. The
corpus evidence suggests that users’ utterances in unconstrained dialogues
contain many instances of acknowledgment. In interpreting these utterances,
identification of the appropriate acknowledgment function affects the state of
the dialogue model and thus plays an important role in determining an
appropriate response by the system. In producing such responses, the
acknowledgment model can provide structurally appropriate utterances. The
fundamental idea is to produce contextually appropriate acknowledgments that
advance the dialogue seamlessly with respect to both domain and control
functions. That is, the system needs to give the right signals at the right
The evidence of the U S WEST VNS corpus suggests that understanding and
production of domain and control utterances are closely linked; they thus
cannot be implemented as independent mechanisms in an SLS. For example, giving
directions involves presenting large amounts of information for which an
installment approach often proved effective. Typically the user was given the
opportunity to choose the style of presentation of directions, either step-by-step
or all at once. The choice of presentation method by the conversants was a
dynamic one: in cases where it became apparent that the user was experiencing
difficulties with either hearing or understanding directions, the wizard often
resorted to the step-by-step approach. This form of repair changed the process
of interaction so that the comprehension of each installment was verified before
proceeding with the next. The conversants in the VNS corpus displayed
relatively higher rates of use of acknowledgment in repair situations or when
unplanned events arose (e.g., the user had gotten lost). Intuitively, people
make more effort to establish mutual knowledge when it is apparent that
miscommunication has occurred than at other times; their certainty criterion
for mutuality (Clark & Marshall, 1981) is raised as a result of the need
for repair. This suggests that a facility for acknowledgment is an important
element in the development of robust SLSs because use of acknowledgment is most
critical precisely when the conversation has gone awry.
We are currently developing a computational model of acknowledgment
based on the empirical work presented in this paper. This model is intended for
integration into a SLS where it will serve both to predict when acknowledgments
are appropriate from the system and when to expect acknowledgments from the
user. Briefly, determining the applicability of an acknowledgment involves
interpreting exchanges in terms of speech acts and then mapping these
speech-act patterns onto the acknowledgment classes described. This, we
believe, will facilitate improved SLS robustness through achievement of a
greater degree of mutual understanding and provide a more natural and intuitive
interaction. The utility and implementation of the empirical model will be the
focus of a later paper.
This work was supported by US WEST Advanced Technologies and the Oregon
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1. A notable exception is the self-acknowledgment which will be
2. The notation for structural class names indicates turns delimited by
arrows (!). Acts combined within a turn are joined with a plus (+) symbol.
Other and self refer to non-acknowledgment acts by the respective conversants. “Self”
refers to the party producing the acknowledgment; “Other” is the other party.
3. The mrequest and mdirect acts are forms of meta-act in which the
speaker initiates a clarification subdialogue or otherwise explicitly addresses
the control of the conversation; mrequest and mdirect are extensions of
Wierzbicka’s (1987) speech-act categories following Novick’s (1988) meta-act
4. In the examples, the acknowledgment of principal interest is
5 All examples are extracted from a
corpus of telephone dialogues from a task-oriented “Wizard-of-Oz” protocol
collection study described in Section 3.1. The examples in this paper are
notated with the corpus dialogue reference number and each turn is numbered for
purposes of reference.
This paper was published as Novick, D., and Sutton, S.
(1994). An empirical model of acknowledgment for spoken-language systems, Proceedings of ACL-94, Las Cruces, NM,
June, 1994, 96-101.
DGN, November 23, 2000