AN EMPIRICAL MODEL OF ACKNOWLEDGMENT 
FOR SPOKEN-LANGUAGE SYSTEMS 
David 6:. Novick and Stephen Sutton 
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 
novick~cse, ogi. edu 
Abstract 
We refine and extend prior views of the descrip- 
tion, purposes, and contexts-of-use of acknowledg- 
ment acts through empirical examination of the 
use of acknowledgments in task-based conversa- 
tion. We distinguish three broad classes of ac- 
knowledgments (other--*ackn, self--*other--*ackn, 
and self+ackn) and present a catalogue of 13 pat- 
terns within these classes that account for the spe- 
cific uses of acknowledgment in the corpus. 
1 MOTIVATION 
This study is motivated by the need for better di- 
alogue models in spoken-language systems (SLSs). 
Dialogue models contribute directly to the interac- 
tion by providing inter-utterance coherence. Flu- 
ent understanding and use of acknowledgments 
should improve spoken-language systems in at 
least the following ways: 
• Preventing miscommunication. Acknowledg- 
ments are an important device for establishing 
mutual knowledge and signaling comprehension. 
Early detection and correction of cases of mis- 
communication 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 "nat- 
uralness" 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 cen- 
tral 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 di- 
alogue 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 examina- 
tion 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 
conversations. 
2 RELATED WORK 
Clark and Schaefer (1989) suggested that acknowl- 
edgments 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 presen- 
tation. These include continued attention, initi- 
ation of the next relevant contribution, acknowl- 
edgment, demonstration, and display. 
Thus acknowledgments are common linguistic 
devices used to provide feedback. Broadly speak- 
ing, acknowledgments are responsive acts. 1 That 
is, they are usually uttered in (possibly partial) 
response to a production by another speaker; ac- 
knowledgment 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 acknowl- 
edgment act is generally the perception of mutu- 
ality of some belief. 
1A notable exception is the self-acknowledgment 
which will be discussed shortly 
96 
In previous research, the function of acknowl- 
edgments 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 dia- 
logues and promote "smooth" conversations (Dun- 
can and 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 con- 
tinued attention and to display the recipient's un- 
derstanding that the speaker is in an extended 
turn that is not yet complete. Moreover, con- 
tinuers indicate the turning down of an oppor- 
tunity to undertake a repair subdialogue regard- 
ing the previous utterance or to initiate a new 
turn. Assessments--such as "oh wow" and "gosh, 
really?"-- are essentially an elaboration on con- 
tinuers. That is, they occur in much the same 
environment and have similar properties to con- 
tinuers, but in addition express a brief assessment 
of the previous utterance. 
Empirical analysis of conversations has in- 
dicated 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 particu- 
lar, 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 com- 
pletion. 
Clearly, acknowledgments are an important 
device for providing evidence of understanding and 
for avoiding miscommunication between parties. 
Just as next-relevant-contributions include the en- 
tire range of potential task or domain actions, the 
task-based role of acknowledgments can be differ- 
entiated 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 co- 
herence of multi-utterance contributions. 
3 DIALOGUE ANALYSIS 
In this section, we describe the task characteris- 
tics and the corpus used for this study, present 
a theoretical model of acknowledgment acts in 
task-based dialogue, and present an analysis of ac- 
knowledgment acts based on corpus material. 
3.1 THE VEHICLE NAVIGATION 
SYSTEM CORPUS 
The corpus we analyzed was collected by U S 
WEST Advanced Technologies in the domain of 
a vehicle navigation system (VNS). A VNS is in- 
tended 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 con- 
trol. U S WEST collected the dialogues through 
a Wizard-of-Oz experiment (Brunner et M., 1992) 
in which the wizard was free to engage in linguis- 
tically unconstrained interaction in the VNS task. 
Each of the 21 subjects performed three tasks in 
the VNS domain. As a whole, the corpus con- 
tained 2499 turns and 1107 acknowledgments. 
3.2 A TASK-BASED MODEL OF 
ACKNOWLEDGMENT ACTS 
The generally accepted view of acknowledg- 
ments, as noted earlier, distinguishes between 
two classes--namely continuers and assessments 
(Schegloff, 1982). Indeed, there were many oc- 
currences 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 sev- 
eral instances of acknowledgment being used at 
the beginning of a turn by the same speaker. This 
contrasts with the notions of continuers and as- 
sessments which, by definition, occur as unitary 
productions in the context of extended turns by 
another speaker. Clearly, an acknowledgment oc- 
curring at the beginning of a turn is not serving 
as a prompt for the other speaker to continue. 
To account for the evidence provided by the 
VNS corpus, we propose to extend Schegloff's clas- 
sification scheme into a task-based model of ac- 
knowledgment acts. This model formalizes the 
meaning and usage characteristics of acknowledg- 
ments, based On an analysis of what acknowledg- 
ments mean and when acknowledgments are used 
in the VNS dialogues. 
A useful way of looking at the role of acknowl- 
edgments 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 and Sacks, 1973; Clark and Schae- 
97 
fer, 1989). An adjacency pair is formed by two 
consecutive utterances that have a canonical re- 
lationship, such as question-answer and greeting- 
greeting. An acknowledgment can be produced as 
the second phase of an adjacency pair or follow- 
ing 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 adja- 
cency 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; 
• Sclf--*other--*ackn, where Self initiates an ex- 
change, Other (eventually) completes the ex- 
change, and Self then utters an acknowledg- 
ment; and 
• Self÷ackn, where Self includes an acknowledg- 
ment in an utterance outside of an adjacency 
pair. 
In the other--*ackn class, the exchange is a ba- 
sic 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 
In this section, we elaborate the structural classes 
of acknowledgment through a catalogue of pat- 
terns 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 acknowl- 
edgments 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 embed- 
2The 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 respec- 
tive conversants. "Self" refers to the party producing 
the acknowledgment; "Other" is the other party. 
ded exchanges, in which basic patterns are used 
to build more complex patterns. 
3.3.1 Other-*Aekn Acknowledgments in the 
other--*ackn class relate principally to the im- 
mediately antecedent utterance as opposed to 
the prior exchange, which is covered by the 
self-*other-*ackn class. In Clark and Schae- 
fer'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 axe 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 sec- 
ond turn is Self's acknowledgment. In some cases, 
Self's turn also extends to include other signifi- 
cant utterances. We illustrate the other-*ackn 
class through examination of the inform-*ackn 
and inform-*ackn+mrequest patterns. 
Inform-*Aekn 
The inform-*aekn pattern covers cases where 
Other performs an inform act and Self responds 
with an acknowledgment of that act. In the follow- 
ing example 4 of an inform-* ackn, the wizard gives 
directions to the user, who acknowledges these di- 
rections. This is an example of an acknowledg- 
ment that Schegloff (1982) would call a continuer. 
Example 1 (U6.3.1) 5 
(1.1) Wizard: On Evans, you need to turn 
left and head West for approximately three 
quarters of a mile to Clermont. 
(1.2) User: Okay. 
(1.3) Wizard: And, urn, on Clermont you 
turn left, heading South for about two 
blocks to Iliff. 
Here, the "okay" at turn 1.2 indicates the 
user's acceptance of the wizaxd'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 acknowledg- 
3The mrequest and redirect acts are forms of meta- 
act in which the speaker initiates a clarification subdi- 
alogue or otherwise explicitly addresses the control of 
the conversation; rarequest and redirect are extensions 
of Wierzbicka's (1987) speech-act categories following 
Novick's (1988) meta-act analysis. 
4In the examples, the acknowledgment of principal 
interest is highlighted. 
~All examples are extracted from a corpus of tele- 
phone 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 di- 
alogue reference number and each turn is numbered 
for purposes of reference. 
98 
Other--~ Ackn \[ Self--~Other--~ Ackn Self÷ Ackn 
inform~ackn 
inform--*ackn-bmrequest 
request--*ackn+inform 
mdirect-*ackn 
preclose--*ackn 
inform~ackn-*ackn 
request--*inform---~ackn 
mrequest--*inform--~ackn 
mdirect--~ackn--*ackn 
inform+ackn+inform 
mrequest+ackn 
mdirect+ackn 
Table 1: A Summary of Speech-Act Patterns for Structural Classes of Acknowledgment 
ment 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 infor- 
mation 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. 
Inform--* Ackn+ MRequest 
The inform--*ackn+mrequest class repre- 
sents 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 di- 
alogue 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) 
(2.1) Wizard: Okay. Then you want to go 
north on Speer Boulevard for one and one 
half miles to Alcott Street. 
(2.1) User: Okay. I want to go right on 
Speer? 
(2.2) 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 origi- 
nal inform act but which is potentially available-- 
namely "right" instead of "north." 
3.3.2 Self--~Other---~Ackn Acknowledgments 
in the self--~other--*ackn class relate to the pre- 
vious exchange, rather than just the previous ut- 
terance. Broadly speaking, they express the cur- 
rent state of the dialogue in addition to embody- 
ing the functionality of other--~ackn acknowledg- 
ments. That is, they explicitly mark the com- 
pletion 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 under- 
standing and acceptance of both the previous ex- 
change and the previous utterance. The canon- 
ical patterns of the self--* other--* ackn class, as 
listed in Table 1, include inform--*ackn---~ackn, 
request-* inform-~ ackn, mrequest-* inf orm--~ ackn 
and mdirect--~ackn--*ackn. We illustrate the 
self--*other--~ackn class through examination of 
the request-~inform--, ackn pattern. 
Request --*Inform--~Aekn 
The request--~inform--*ackn class covers cases 
where Self requests an inform act of Other. Other 
then performs that inform act and Self acknowl- 
edges. Note that the acknowledgment in this case 
follows a completed request-inform adjacency pair. 
Earlier, we mentioned that question-answer adja- 
cency 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 re- 
quest by communicating the desired information 
and the wizard then acknowledges. Here the ac- 
knowledgment 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 re- 
quest initiated by the wizard at 3.1 has been sat- 
isfied and thus the exchange is complete. 
Example 3 (U2.1.1) 
(3.1) Wizard: Okay and uh, what's your 
starting location? 
(3.2) User: I'm at 36th and Sheridan at the 
Park-n-Ride. 
(3.3) Wizard: Okay, one moment please. 
3.3.3 Self-bAckn 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 establish- 
ing mutual beliefs. The canonical patterns, as 
99 
listed in Table 1, include inform+ackn+inform, 
mrequest+ackn, and mdirect+ackn. We illustrate 
the self+ackn class through examination of the in- 
form +ackn +inform pattern. 
Inform+Ackn+Inform 
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, urn, the first 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 intona- 
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. 
3.3.4 Embedded Exchanges We noted earlier 
that basic patterns can used to build more com- 
plex patterns. This can lead potentially to vari- 
ations in patterns of acknowledgments. In par- 
ticular, it is possible to observe cascades of ac- 
knowledgments 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 and 
acknowledgments. In this section we will consider 
one of the variations encountered in the VNS cor- 
pus; where an {nform---*ackn--~ackn replaces the 
inform act in an inform--*ackn sequence. In the 
following example, there are three successive ac- 
knowledgment 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 ac- 
ceptance of the wizard's inform act in which a di- 
rection 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 acknowl- 
edgment, issued by the user at 5.4, is associated 
with the user recognizing that the wizard has fin- 
ished clarifying directions; the user thus acknowl- 
edges this embedded inform act. The user then 
indicates satisfaction and approval of the wizard's 
directions with the assessment "Sounds good." 
Example 5 (U6.2.1) 
(5.1) Wizard: Okay, it was, urn, on Evans 
it's three and three quarter miles to Jas- 
mine. 
(5.2) User: Three, okay. 
(5.3) Wizard: Okay. 
(5.4) User: All right, sounds good. 
4 CONCLUSION 
Why is a conversation-analytic study of acknowl- 
edgment useful in the development of spoken 
language systems? SLSs developers face the 
dual challenges of creating both domain-based 
dialogues and repair-oriented dialogues. Lack- 
ing systematic mechanisms for natural mainte- 
nance of mutuality, SLSs tend to rely on do- 
main structures--producing rather stolid interac- 
tion. The most advanced systems incorporate re- 
pair acts, but are unable to relate the repairs to the 
main dialogue structures in a natural way. The ac- 
knowledgment model described in this paper pro- 
vides a systematic method of maintaining mutu- 
ality 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 interpret- 
ing these utterances, identification of the appro- 
priate 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 acknowl- 
edgment model can provide structurally appropri- 
ate utterances. The fundamental idea is to pro- 
duce contextually appropriate acknowledgments 
that advances the dialogue seamlessly with respect 
to both domain and control functions. That is, the 
system needs to give the right signals at the right 
time. 
The evidence of the U S WEST VNS cor- 
pus 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 direc- 
tions involves presenting large amounts of infor- 
mation for which an installment approach often 
proved effective. Typically the user was given the 
opportunity to choose the style of presentation of 
100 
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 ap- 
parent that the user was experiencing difficulties 
with either hearing or understanding directions, 
the wizard often resorted to the step-by-step ap- 
proach. This form of repair changed the process of 
interaction so that the comprehension of each in- 
stallment 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 cri- 
terion for mutuality (Clark and 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 crit- 
ical 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 in- 
tended 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, deter- 
mining the applicability of an acknowledgment in- 
volves 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 in- 
tuitive interaction. The utility and implementa- 
tion of the empirical model will be the focus of a 
later paper. 
5 ACKNOWLEDGMENTS 
This work was supported by U S WEST Advanced 
Technologies and the Oregon Advanced Comput- 
ing Institute. 

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