Proceedings of the 43rd Annual Meeting of the ACL, pages 50–57,
Ann Arbor, June 2005. c©2005 Association for Computational Linguistics
Aggregation improves learning:
experiments in natural language generation for intelligent tutoring systems
Barbara Di Eugenio and Davide Fossati and Dan Yu
University of Illinois
Chicago, IL, 60607, USA
{bdieugen,dfossa1,dyu6}@uic.edu
Susan Haller
University of Wisconsin - Parkside
Kenosha, WI 53141, USA
haller@cs.uic.edu
Michael Glass
Valparaiso University
Valparaiso, IN, 46383, USA
Michael.Glass@valpo.edu
Abstract
To improve the interaction between students
and an intelligent tutoring system, we devel-
oped two Natural Language generators, that we
systematically evaluated in a three way com-
parison that included the original system as
well. We found that the generator which intu-
itively produces the best language does engen-
der the most learning. Specifically, it appears
that functional aggregation is responsible for
the improvement.
1 Introduction
The work we present in this paper addresses three
issues: evaluation of Natural Language Generation
(NLG) systems, the place of aggregation in NLG,
and NL interfaces for Intelligent Tutoring Systems.
NLG systems have been evaluated in various
ways, such as via task efficacy measures, i.e., mea-
suring how well the users of the system perform on
the task at hand (Young, 1999; Carenini and Moore,
2000; Reiter et al., 2003). We also employed task
efficacy, as we evaluated the learning that occurs
in students interacting with an Intelligent Tutoring
System (ITS) enhanced with NLG capabilities. We
focused on sentence planning, and specifically, on
aggregation. We developed two different feedback
generation engines, that we systematically evaluated
in a three way comparison that included the orig-
inal system as well. Our work is novel for NLG
evaluation in that we focus on one specific com-
ponent of the NLG process, aggregation. Aggrega-
tion pertains to combining two or more of the mes-
sages to be communicated into one sentence (Reiter
and Dale, 2000). Whereas it is considered an es-
sential task of an NLG system, its specific contri-
butions to the effectiveness of the text that is even-
tually produced have rarely been assessed (Harvey
and Carberry, 1998). We found that syntactic aggre-
gation does not improve learning, but that what we
call functional aggregation does. Further, we ran a
controlled data collection in order to provide a more
solid empirical base for aggregation rules than what
is normally found in the literature, e.g. (Dalianis,
1996; Shaw, 2002).
As regards NL interfaces for ITSs, research on the
next generation of ITSs (Evens et al., 1993; Litman
et al., 2004; Graesser et al., 2005) explores NL as
one of the keys to bridging the gap between cur-
rent ITSs and human tutors. However, it is still not
known whether the NL interaction between students
and an ITS does in fact improve learning. We are
among the first to show that this is the case.
We will first discuss DIAG, the ITS shell we are
using, and the two feedback generators that we de-
veloped, DIAG-NLP1and DIAG-NLP2. Since the
latter is based on a corpus study, we will briefly de-
scribe that as well. We will then discuss the formal
evaluation we conducted and our results.
2 Natural Language Generation for DIAG
DIAG (Towne, 1997) is a shell to build ITSs based
on interactive graphical models that teach students to
troubleshoot complex systems such as home heating
and circuitry. A DIAG application presents a student
with a series of troubleshooting problems of increas-
ing difficulty. The student tests indicators and tries
to infer which faulty part (RU) may cause the abnor-
mal states detected via the indicator readings. RU
stands for replaceable unit, because the only course
of action for the student to fix the problem is to re-
place faulty components in the graphical simulation.
50
Figure 1: The furnace system
Fig. 1 shows the furnace, one subsystem of the home
heating system in our DIAG application. Fig. 1 in-
cludes indicators such as the gauge labeled Water
Temperature, RUs, and complex modules (e.g., the
Oil Burner) that contain indicators and RUs. Com-
plex components are zoomable.
At any point, the student can consult the tutor
via the Consult menu (cf. the Consult button in
Fig. 1). There are two main types of queries: Con-
sultInd(icator) and ConsultRU. ConsultInd queries
are used mainly when an indicator shows an ab-
normal reading, to obtain a hint regarding which
RUs may cause the problem. DIAG discusses the
RUs that should be most suspected given the symp-
toms the student has already observed. ConsultRU
queries are mainly used to obtain feedback on the di-
agnosis that a certain RU is faulty. DIAG responds
with an assessment of that diagnosis and provides
evidence for it in terms of the symptoms that have
been observed relative to that RU.
The original DIAG system (DIAG-orig) uses very
simple templates to assemble the text to present to
the student. The top parts of Figs. 2 and 3 show the
replies provided by DIAG-orig to a ConsultInd on
the Visual Combustion Check, and to a ConsultRu
on the Water Pump.
The highly repetitive feedback by DIAG-orig
screams for improvements based on aggregation
techniques. Our goal in developing DIAG-NLP1
and DIAG-NLP2 was to assess whether simple,
rapidly deployable NLG techniques would lead to
measurable improvements in the student’s learning.
Thus, in both cases it is still DIAG that performs
content determination, and provides to DIAG-NLP1
and DIAG-NLP2 a file in which the facts to be com-
municated are written – a fact is the basic unit of
information that underlies each of the clauses in a
reply by DIAG-orig. The only way we altered the
interaction between student and system is the ac-
tual language that is presented in the output win-
dow. In DIAG-NLP1 we mostly explored using syn-
tactic aggregation to improve the feedback, whereas
DIAG-NLP2 is corpus-based and focuses on func-
tional aggregation. In both DIAG-NLP1 and DIAG-
NLP2, we use EXEMPLARS (White and Cald-
well, 1998), an object-oriented, rule-based genera-
tor. The rules (called exemplars) are meant to cap-
ture an exemplary way of achieving a communica-
tive goal in a given context. EXEMPLARS selects
rules by traversing the exemplar specialization hi-
erarchy and evaluating the applicability conditions
associated with each exemplar.
The visual combustion check is igniting which is abnormal
(normal is combusting).
Oil Nozzle always
produces this abnormality when it fails.
Oil Supply Valve always
produces this abnormality when it fails.
Oil pump always
produces this abnormality when it fails.
Oil Filter always
produces this abnormality when it fails.
System Control Module sometimes
produces this abnormality when it fails.
Ignitor Assembly never
produces this abnormality when it fails.
Burner Motor always
produces this abnormality when it fails.
The visual combustion check indicator is igniting.
This is abnormal.
Normal is combusting.
Within the furnace system,
this is sometimes caused if
the System Control Module has failed.
Within the Oil Burner
this is never caused if
the Ignitor Assembly has failed.
In contrast, this is always caused if
the Burner Motor, Oil Filter, Oil Pump,
Oil Supply Valve, or Oil Nozzle has failed.
The combustion is abnormal.
In the oil burner, check the units along the path of the oil and
the burner motor.
Figure 2: Answers to ConsultInd by DIAG-orig,
DIAG-NLP1and DIAG-NLP2
51
Water pump is a very poor suspect.
Some symptoms you have seen conflict with that theory.
Water pump sound was normal.
This normal indication never results when this unit fails.
Visual combustion check was igniting.
This abnormal indication never results when this unit fails.
Burner Motor RMP Gauge was 525.
This normal indication never results when this unit fails.
The Water pump is a very poor suspect.
Some symptoms you have seen conflict with that theory.
The following indicators never display normally
when this unit fails.
Within the furnace system,
the Burner Motor RMP Gauge is 525.
Within the water pump and safety cutoff valve,
the water pump sound indicator is normal.
The following indicators never display abnormally
when this unit fails.
Within the fire door sight hole,
the visual combustion check indicator is igniting.
The water pump is a poor suspect since the water pump
sound is ok.
You have seen that the combustion is abnormal.
Check the units along the path of the oil and the electrical
devices.
Figure 3: Answers to ConsultRu by DIAG-orig,
DIAG-NLP1 and DIAG-NLP2
2.1 DIAG-NLP1: Syntactic aggregation
DIAG-NLP11 (i) introduces syntactic aggregation
(Dalianis, 1996; Huang and Fiedler, 1996; Reape
and Mellish, 1998; Shaw, 2002) and what we call
structural aggregation, namely, grouping parts ac-
cording to the structure of the system; (ii) gener-
ates some referring expressions; (iii) models a few
rhetorical relations; and (iv) improves the format of
the output.
The middle parts of Figs. 2 and 3 show the revised
output produced by DIAG-NLP1. E.g., in Fig. 2 the
RUs of interest are grouped by the system modules
that contain them (Oil Burner and Furnace System),
and by the likelihood that a certain RU causes the
observed symptoms. In contrast to the original an-
swer, the revised answer highlights that the Ignitor
Assembly cannot cause the symptom.
In DIAG-NLP1, EXEMPLARS accesses the
SNePS Knowledge Representation and Reasoning
System (Shapiro, 2000) for static domain informa-
tion.2 SNePS makes it easy to recognize structural
1DIAG-NLP1 actually augments and refines the first feed-
back generator we created for DIAG, DIAG-NLP0 (Di Eugenio
et al., 2002). DIAG-NLP0 only covered (i) and (iv).
2In DIAG, domain knowledge is hidden and hardly acces-
similarities and use shared structures. Using SNePS,
we can examine the dimensional structure of an ag-
gregation and its values to give preference to aggre-
gations with top-level dimensions that have fewer
values, to give summary statements when a dimen-
sion has many values that are reported on, and to
introduce simple text structuring in terms of rhetor-
ical relations, inserting relations like contrast and
concession to highlight distinctions between dimen-
sional values (see Fig. 2, middle).
DIAG-NLP1 uses the GNOME algorithm (Kib-
ble and Power, 2000) to generate referential expres-
sions. Importantly, using SNePS propositions can
be treated as discourse entities, added to the dis-
course model and referred to (see This is ... caused
if ... in Fig. 2, middle). Information about lexical
realization, and choice of referring expression is en-
coded in the appropriate exemplars.
2.2 DIAG-NLP2: functional aggregation
In the interest of rapid prototyping, DIAG-NLP1
was implemented without the benefit of a corpus
study. DIAG-NLP2 is the empirically grounded
version of the feedback generator. We collected
23 tutoring interactions between a student using the
DIAG tutor on home heating and two human tutors,
for a total of 272 tutor turns, of which 235 in re-
ply to ConsultRU and 37 in reply to ConsultInd (the
type of student query is automatically logged). The
tutor and the student are in different rooms, sharing
images of the same DIAG tutoring screen. When
the student consults DIAG, the tutor sees, in tabular
form, the information that DIAG would use in gen-
erating its advice — the same “fact file” that DIAG
gives to DIAG-NLP1and DIAG-NLP2— and types
a response that substitutes for DIAG’s. The tutor is
presented with this information because we wanted
to uncover empirical evidence for aggregation rules
in our domain. Although we cannot constrain the tu-
tor to mention only the facts that DIAG would have
communicated, we can analyze how the tutor uses
the information provided by DIAG.
We developed a coding scheme (Glass et al.,
2002) and annotated the data. As the annotation was
performed by a single coder, we lack measures of
intercoder reliability. Thus, what follows should be
taken as observations rather than as rigorous find-
ings – useful observations they clearly are, since
sible. Thus, in both DIAG-NLP1 and DIAG-NLP2 we had to
build a small knowledge base that contains domain knowledge.
52
DIAG-NLP2 is based on these observations and its
language fosters the most learning.
Our coding scheme focuses on four areas. Fig. 4
shows examples of some of the tags (the SCM is the
System Control Module). Each tag has from one to
five additional attributes (not shown) that need to be
annotated too.
Domain ontology. We tag objects in the domain
with their class indicator, RU and their states, de-
noted by indication and operationality, respectively.
Tutoring actions. They include (i) Judgment. The
tutor evaluates what the student did. (ii) Problem
solving. The tutor suggests the next course of ac-
tion. (iii) The tutor imparts Domain Knowledge.
Aggregation. Objects may be functional aggre-
gates, such as the oil burner, which is a system com-
ponent that includes other components; linguistic
aggregates, which include plurals and conjunctions;
or a summary over several unspecified indicators or
RUs. Functional/linguistic aggregate and summary
tags often co-occur, as shown in Fig. 4.
Relation to DIAG’s output. Contrary to all other
tags, in this case we annotate the input that DIAG
gave the tutor. We tag its portions as included / ex-
cluded / contradicted, according to how it has been
dealt with by the tutor.
Tutors provide explicit problem solving directions
in 73% of the replies, and evaluate the student’s ac-
tion in 45% of the replies (clearly, they do both in
28% of the replies, as in Fig. 4). As expected, they
are much more concise than DIAG, e.g., they never
mention RUs that cannot or are not as likely to cause
a certain problem, such as, respectively, the ignitor
assembly and the SCM in Fig. 2.
As regards aggregation, 101 out of 551 RUs, i.e.
18%, are labelled as summary; 38 out of 193 indica-
tors, i.e. 20%, are labelled as summary. These per-
centages, though seemingly low, represent a consid-
erable amount of aggregation, since in our domain
some items have very little in common with others,
and hence cannot be aggregated. Further, tutors ag-
gregate parts functionally rather than syntactically.
For example, the same assemblage of parts, i.e., oil
nozzle, supply valve, pump, filter, etc., can be de-
scribed as the other items on the fuel line or as the
path of the oil flow.
Finally, directness – an attribute on the indica-
tor tag – encodes whether the tutor explicitly talks
about the indicator (e.g., The water temperature
gauge reading is low), or implicitly via the object
to which the indicator refers (e.g., the water is too
cold). 110 out of 193 indicators, i.e. 57%, are
marked as implicit, 45, i.e. 41%, as explicit, and 2%
are not marked for directness (the coder was free to
leave attributes unmarked). This, and the 137 occur-
rences of indication, prompted us to refer to objects
and their states, rather than to indicators (as imple-
mented by Steps 2 in Fig. 5, and 2(b)i, 3(b)i, 3(c)i in
Fig. 6, which generate The combustion is abnormal
and The water pump sound is OK in Figs. 2 and 3).
2.3 Feedback Generation in DIAG-NLP2
In DIAG-NLP1 the fact file provided by DIAG is
directly processed by EXEMPLARS. In contrast, in
DIAG-NLP2 a planning module manipulates the in-
formation before passing it to EXEMPLARS. This
module decides which information to include ac-
cording to the type of query the system is respond-
ing to, and produces one or more Sentence Structure
objects. These are then passed to EXEMPLARS
that transforms them into Deep Syntactic Structures.
Then, a sentence realizer, RealPro (Lavoie and Ram-
bow, 1997), transforms them into English sentences.
Figs. 5 and 6 show the control flow in DIAG-
NLP2 for feedback generation for ConsultInd and
ConsultRU. Step 3a in Fig. 5 chooses, among all
the RUs that DIAG would talk about, only those
that would definitely result in the observed symp-
tom. Step 2 in the AGGREGATE procedure in Fig. 5
uses a simple heuristic to decide whether and how to
use functional aggregation. For each RU, its possi-
ble aggregators and the number n of units it covers
are listed in a table (e.g., electrical devices covers
4 RUs, ignitor, photoelectric cell, transformer and
burner motor). If a group of REL-RUs contains k
units that a certain aggregator Agg covers, if k < n2 ,
Agg will not be used; if n2 ≤ k < n, Agg preceded
by some of will be used; if k = n, Agg will be used.
DIAG-NLP2 does not use SNePS, but a relational
database storing relations, such as the ISA hierarchy
(e.g., burner motor IS-A RU), information about ref-
erents of indicators (e.g., room temperature gauge
REFERS-TO room), and correlations between RUs
and the indicators they affect.
3 Evaluation
Our empirical evaluation is a three group, between-
subject study: one group interacts with DIAG-orig,
53
[judgment [replaceable−unit the ignitor] is a poor suspect] since [indication combustion is working] during startup. The problem is
that the SCM is shutting the system off during heating.
[domain−knowledge The SCM reads [summary [linguistic−aggregate input signals from sensors]] and uses the signals to determine
how to control the system.]
[problem−solving Check the sensors.]
Figure 4: Examples of a coded tutor reply
1. IND←queried indicator
2. Mention the referent of IND and its state
3. IF IND reads abnormal,
(a) REL-RUs←choose relevant RUs
(b) AGGR-RUs←AGGREGATE(REL-RUs)
(c) Suggest to check AGGR-RUs
AGGREGATE(RUs)
1. Partition REL-RUs into subsets by system structure
2. Apply functional aggregation to subsets
Figure 5: DIAG-NLP2: Feedback generation for
ConsultInd
one with DIAG-NLP1, one with DIAG-NLP2. The
75 subjects (25 per group) were all science or engi-
neering majors affiliated with our university. Each
subject read some short material about home heat-
ing, went through one trial problem, then continued
through the curriculum on his/her own. The curricu-
lum consisted of three problems of increasing dif-
ficulty. As there was no time limit, every student
solved every problem. Reading materials and cur-
riculum were identical in the three conditions.
While a subject was interacting with the system,
a log was collected including, for each problem:
whether the problem was solved; total time, and time
spent reading feedback; how many and which in-
dicators and RUs the subject consults DIAG about;
how many, and which RUs the subject replaces. We
will refer to all the measures that were automatically
collected as performance measures.
At the end of the experiment, each subject was ad-
ministered a questionnaire divided into three parts.
The first part (the posttest) consists of three ques-
tions and tests what the student learned about the
domain. The second part concerns whether subjects
remember their actions, specifically, the RUs they
replaced. We quantify the subjects’ recollections in
terms of precision and recall with respect to the log
that the system collects. We expect precision and re-
call of the replaced RUs to correlate with transfer,
namely, to predict how well a subject is able to ap-
ply what s/he learnt about diagnosing malfunctions
1. RU←queried RU
REL-IND←indicator associated to RU
2. IF RU warrants suspicion,
(a) state RU is a suspect
(b) IF student knows that REL-IND is abnormal
i. remind him of referent of REL-IND and
its abnormal state
ii. suggest to replace RU
(c) ELSE suggest to check REL-IND
3. ELSE
(a) state RU is not a suspect
(b) IF student knows that REL-IND is normal
i. use referent of REL-IND and its normal state
to justify judgment
(c) IF student knows of abnormal indicators OTHER-INDs
i. remind him of referents of OTHER-INDs
and their abnormal states
ii. FOR each OTHER-IND
A. REL-RUs←RUs associated with OTHER-IND
B. AGGR-RUs←AGGREGATE(REL-RUs)
∪AGGR-RUs
iii. Suggest to check AGGR-RUs
Figure 6: DIAG-NLP2: Feedback generation for
ConsultRU
to new problems. The third part concerns usability,
to be discussed below.
We found that subjects who used DIAG-NLP2
had significantly higher scores on the posttest, and
were significantly more correct (higher precision)
in remembering what they did. As regards perfor-
mance measures, there are no so clear cut results.
As regards usability, subjects prefer DIAG-NLP1/2
to DIAG-orig, however results are mixed as regards
which of the two they actually prefer.
In the tables that follow, boldface indicates sig-
nificant differences, as determined by an analysis of
variance performed via ANOVA, followed by post-
hoc Tukey tests.
Table 1 reports learning measures, average across
the three problems. DIAG-NLP2 is significantly
better as regards PostTest score (F = 10.359,p =
0.000), and RU Precision (F = 4.719,p =
0.012). Performance on individual questions in the
54
DIAG-orig DIAG-NLP1 DIAG-NLP2
PostTest 0.72 0.69 0.90
RU Precision 0.78 0.70 0.91
RU Recall .53 .47 .40
Table 1: Learning Scores
Figure 7: Scores on PostTest questions
PostTest3 is illustrated in Fig. 7. Scores in DIAG-
NLP2 are always higher, significantly so on ques-
tions 2 and 3 (F = 8.481,p = 0.000, and F =
7.909,p = 0.001), and marginally so on question 1
(F = 2.774,p = 0.069).4
D-Orig D-NLP1 D-NLP2
Total Time 30’17” 28’34” 34’53”
RU Replacements 8.88 11.12 11.36
ConsultInd 22.16 6.92 28.16
Avg. Reading Time 8” 14” 2”
ConsultRU 63.52 45.68 52.12
Avg. Reading Time 5” 4” 5”
Table 2: Performance Measures
Table 2 reports performance measures, cumula-
tive across the three problems, other than average
reading times. Subjects don’t differ significantly in
the time they spend solving the problems, or in the
number of RU replacements they perform. DIAG’s
assumption (known to the subjects) is that there is
only one broken RU per problem, but the simula-
tion allows subjects to replace as many as they want
without any penalty before they come to the correct
solution. The trend on RU replacements is opposite
what we would have hoped for: when repairing a
real system, replacing parts that are working should
clearly be kept to a minimum, and subjects replace
3The three questions are: 1. Describe the main subsystems
of the furnace. 2. What is the purpose of (a) the oil pump (b)
the system control module? 3. Assume the photoelectric cell is
covered with enough soot that it could not detect combustion.
What impact would this have on the system?
4The PostTest was scored by one of the authors, following
written guidelines.
fewer parts in DIAG-orig.
The next four entries in Table 2 report the number
of queries that subjects ask, and the average time it
takes subjects to read the feedback. The subjects
ask significantly fewer ConsultInd in DIAG-NLP1
(F = 8.905,p = 0.000), and take significantly less
time reading ConsultInd feedback in DIAG-NLP2
(F = 15.266,p = 0.000). The latter result is
not surprising, since the feedback in DIAG-NLP2 is
much shorter than in DIAG-orig and DIAG-NLP1.
Neither the reason not the significance of subjects
asking many fewer ConsultInd of DIAG-NLP1 are
apparent to us – it happens for ConsultRU as well,
to a lesser, not significant degree.
We also collected usability measures. Although
these are not usually reported in ITS evaluations,
in a real setting students should be more willing to
sit down with a system that they perceive as more
friendly and usable. Subjects rate the system along
four dimensions on a five point scale: clarity, useful-
ness, repetitiveness, and whether it ever misled them
(the scale is appropriately arranged: the highest clar-
ity but the lowest repetitiveness receive 5 points).
There are no significant differences on individual
dimensions. Cumulatively, DIAG-NLP2 (at 15.08)
slightly outperforms the other two (DIAG-orig at
14.68 and DIAG-NLP1 at 14.32), however, the dif-
ference is not significant (highest possible rating is
20 points).
prefer neutral disprefer
DIAG-NLP1 to DIAG-orig 28 5 17
DIAG-NLP2 to DIAG-orig 34 1 15
DIAG-NLP2 to DIAG-NLP1 24 1 25
Table 3: User preferences among the three systems
prefer neutral disprefer
Consult Ind. 8 1 16
Consult RU 16 0 9
Table 4: DIAG-NLP2 versus DIAG-NLP1
natural concise clear contentful
DIAG-NLP1 4 8 10 23
DIAG-NLP2 16 8 11 12
Table 5: Reasons for system preference
Finally,5 on paper, subjects compare two pairs of
versions of feedback: in each pair, the first feedback
5Subjects can also add free-form comments. Only few did
55
is generated by the system they just worked with,
the second is generated by one of the other two sys-
tems. Subjects say which version they prefer, and
why (they can judge the system along one or more
of four dimensions: natural, concise, clear, content-
ful). The first two lines in Table 3 show that subjects
prefer the NLP systems to DIAG-orig (marginally
significant, χ2 = 9.49,p < 0.1). DIAG-NLP1
and DIAG-NLP2 receive the same number of pref-
erences; however, a more detailed analysis (Table 4)
shows that subjects prefer DIAG-NLP1 for feed-
back to ConsultInd, but DIAG-NLP2 for feedback
to ConsultRu (marginally significant, χ2 = 5.6,p <
0.1). Finally, subjects find DIAG-NLP2 more nat-
ural, but DIAG-NLP1 more contentful (Table 5,
χ2 = 10.66,p < 0.025).
4 Discussion and conclusions
Our work touches on three issues: aggregation, eval-
uation of NLG systems, and the role of NL inter-
faces for ITSs.
In much work on aggregation (Huang and Fiedler,
1996; Horacek, 2002), aggregation rules and heuris-
tics are shown to be plausible, but are not based on
any hard evidence. Even where corpus work is used
(Dalianis, 1996; Harvey and Carberry, 1998; Shaw,
2002), the results are not completely convincing be-
cause we do not know for certain the content to be
communicated from which these texts supposedly
have been aggregated. Therefore, positing empir-
ically based rules is guesswork at best. Our data
collection attempts at providing a more solid em-
pirical base for aggregation rules; we found that tu-
tors exclude significant amounts of factual informa-
tion, and use high degrees of aggregation based on
functionality. As a consequence, while part of our
rules implement standard types of aggregation, such
as conjunction via shared participants, we also intro-
duced functional aggregation (see conceptual aggre-
gation (Reape and Mellish, 1998)).
As regards evaluation, NLG systems have been
evaluated e.g. by using human judges to assess the
quality of the texts produced (Coch, 1996; Lester
and Porter, 1997; Harvey and Carberry, 1998); by
comparing the system’s performance to that of hu-
mans (Yeh and Mellish, 1997); or through task ef-
ficacy measures, i.e., measuring how well the users
so, and the distribution of topics and of evaluations is too broad
to be telling.
of the system perform on the task at hand (Young,
1999; Carenini and Moore, 2000; Reiter et al.,
2003). The latter kind of studies generally contrast
different interventions, i.e. a baseline that does not
use NLG and one or more variations obtained by pa-
rameterizing the NLG system. However, the evalu-
ation does not focus on a specific component of the
NLG process, as we did here for aggregation.
Regarding the role of NL interfaces for ITSs, only
very recently have the first few results become avail-
able, to show that first of all, students do learn when
interacting in NL with an ITS (Litman et al., 2004;
Graesser et al., 2005). However, there are very few
studies like ours, that evaluate specific features of
the NL interaction, e.g. see (Litman et al., 2004). In
our case, we did find that different features of the NL
feedback impact learning. Although we contend that
this effect is due to functional aggregation, the feed-
back in DIAG-NLP2 changed along other dimen-
sions, mainly using referents of indicators instead of
indicators, and being more strongly directive in sug-
gesting what to do next. Of course, we cannot ar-
gue that our best NL generator is equivalent to a hu-
man tutor – e.g., dividing the number of ConsultRU
and ConsultInd reported in Sec. 2.2 by the number
of dialogues shows that students ask about 10 Con-
sultRus and 1.5 ConsultInd per dialogue when in-
teracting with a human, many fewer than those they
pose to the ITSs (cf. Table 2) (regrettably we did not
administer a PostTest to students in the human data
collection). We further discuss the implications of
our results for NL interfaces for ITSs in a compan-
ion paper (Di Eugenio et al., 2005).
The DIAG project has come to a close. We are
satisfied that we demonstrated that even not overly
sophisticated NL feedback can make a difference;
however, the fact that DIAG-NLP2 has the best lan-
guage and engenders the most learning prompts us
to explore more complex language interactions. We
are pursuing new exciting directions in a new do-
main, that of basic data structures and algorithms.
We are investigating what distinguishes expert from
novice tutors, and we will implement our findings
in an ITS that tutors in this domain.
Acknowledgments. This work is supported by the Office
of Naval Research (awards N00014-99-1-0930 and N00014-00-
1-0640), and in part by the National Science Foundation (award
IIS 0133123). We are grateful to CoGenTex Inc. for making
EXEMPLARS and RealPro available to us.
56
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