Paraphrasing Using Given and New Information 
in a Question-Answer System 
Kathleen R. McKeown 
Department of Computer and Information Science 
The Moore School 
University of Pennsylvania, Philadelphia, Pa. 19104 
ABSTRACT: The design and implementation of a paraphrase 
component for a natural language questlon-answer system 
(CO-OP) is presented. A major point made is the role of 
given and new information in formulating a paraphrase 
that differs in a meaningful way from the user's 
question. A description is also given of the 
transformational grammar used by the paraphraser to 
generate questions. 
I • INTRO~ION 
In a natural language interface to a database query 
system, a paraphraser can be used to ensure that the 
system has correctly understood the user. Such a 
paraphraser has been developed as part of the CO-OP 
system \[ KAPLAN 79\]. In CO-OP, an internal 
representation of the user's question is passed to the 
paraphraser which then generates a new version of the 
question for the user. Upon seeing the paraphrase, the 
user has the option of rephrasing her/his question 
before the system attempts to answer it. Thus, if the 
question was not interpreted correctly, the error can be 
caught before a possibly lengthy search of the database 
is initiated. Furthermore, the user is assured that the 
answer s/he receives is an answer to the question asked 
and not to a deviant version of it. 
The idea of using a paraphraser in the above way is not 
new. To date, other systems have used canned templates 
to form paraphrases, filling in empty slots in the 
pattern with information from the user's question 
\[WALTZ 78; CODD 78\]. In CO-OP, a transformational 
grammar is used to generate the paraphrase from an 
internal representation of the question. Moreover, the 
CO-OP paraphraser generates a question that differs in a 
meaningful way from the original question. It makes use 
of a distinction between given and new information to 
indicate to the user the existential presuppositions 
made In her/his question. 
II. OVERVIEW OF THE CO-OP S~"3-rEM 
The CO-OP system is aimed at infrequent users of 
database query systems. These casual users are likely 
to be unfamiliar with computer systems and unwilling to 
invest the time needed to learn a formal query language. 
Being able to converse naturally in English enables such 
persons to tap the information available in a database. 
In order to allow the question-answer process to proceed 
naturally, CO-OP follows some of the "co-operative 
principles" of conversation \[GRICE 75\]. In particular, 
the system attempts to find meaningful answers to failed 
questions by addressing any incorrect assumptions the 
questioner may have made in her/his question. When the 
direct response to a question would be simply "no" or 
"none", CO-OP gives a more informative response by 
correcting the questloner's mistaken asstm~tlons. 
The false assumptions that CO-OP corrects are the 
existential presuppositions of the question.* Since 
these presuppositions can he computed from the surface 
structure of the question, a large store of semantic 
knowledge for inferenclng purposes is not needed. In 
*For example, in the question "Which users work on 
projects sponsored by NASA?', the speaker makes the 
existential presupposition that there are projects 
mpommred by NASA. 
67 
fact, a lexicon and database schema are the only 
items which contain domain-specific information. 
Consequently, the CO-OP system is a portable one; a 
change of database requires that only these two 
knowledge sources be modified. 
III. THE CO-OP PARAP~%~SER 
CO-OP's paraphraser provides the only means of 
error-checking for the casual user. If the ¢,ser is 
familiar with the system, s/he can ask to have the 
intermediate results printed, in which case the parser's 
output and the formal database query will be shown. The 
naive user however, is unlikely to understand these 
results. It is for this reason that the paraphraser was 
designed to respond in English. 
The use of English to paraphrase queries creates several 
problems. The first is that natural language is 
inherently ambiguous. A paraphrase must clarify the 
system's interpretation of possible ambiguous phrases in 
the question without introducing additional ambiguity. 
One particular type of ambiguity that a paraphraser must 
address is caused by the linear nature of sentences. A 
modifying relative clause, for example, frequently 
cannot be placed directly after the noun phrase it 
modifies. In such cases, the semantics of the sentence 
may indicate the correct choice of modified noun phrase, 
but occasionally,, the sentence may be genuinely 
ambiguouS. For example, question (A) below has two 
interpretations, both equally plausible. The speaker 
could be referring to books dating from the '~0s or to 
computers dating from the '60s. 
(A) Which students read books on computers dating 
from the '60s? 
A second problem in paraphrasing English queries is the 
possibility of generating the exact question that was 
originally asked. If a grammar were developed to simply 
generate English from an underlying representation of 
the question this possibility could be realized. 
Instead, a method must be devised which can determine 
how the phrasing should differ from the original. 
The CO-OF paraphraser addresses both the problem of 
ambiguity and the rephrasing of the question. It makes 
the system's interpretation of the question explicit by 
breaking down the clauses of the question and reordering 
them dependent upon their function in the sentence. 
Thus, questlon (A) above will result in ei ther 
paraphrase (B) or (C), reflecting the interpretation the 
system has chosen. 
(B) Assuming that there are books on computers 
(those computers date from the '60s), which 
students read those books? 
(C) Assuming that there are hooks on computers 
(those hooks date from the '~Os), which students 
read those books? 
~1~e method adopted guarantees that the paraphrase will 
differ from the original except in cases where no 
relative clauses or prepositional phrases were used. It 
was formulated on the basis of a distinction between 
given and new information and indicates to the user the 
presuppositions s/he has made in the question (in the 
"assuming that" clause), while focussing her/his 
attention on the attributes of the class s/he is 
interested in. 
IV. LINGUISTIC 8ACI~ROUND 
As mentioned earlier, the lexicon and the database are 
the sole sources of world knowledqe for CO-OP. While 
this design increases CO-OP's portability, it means that 
little semantic information is available for the 
paraphraser's use. Contextual information is also 
limlte~ since no running history or context is 
maintained for a user session in the current version. 
The input the paraphraser receives from the parser is 
basically a syntactic parse tree of the question. Using 
this information, the paraphraser must reconstruct the 
question to obtain a phrasing different from the 
original. The following question must therefore be 
addressed: 
What reasons are there for choosing one syntactic 
form of expression over another? 
Some linguists maintain that word order is affected by 
functional roles elements play within the sentence.* 
Terminology used to describe the t~pes of roles that can 
occur varies widely. Some of the dlstinctons that have 
been described include given/new, topic/comment, 
theme/theme, and presupposition/focus. Definitions of 
these terms however, are not consistent (for example, 
see \[PRINCE ?9\] for a discussion of various usages of 
"given/new" ). 
Nevertheless, one influence on expression does appear to 
be the interaction of sentence content and the beliefs 
of the speaker concerning the knowledge of the listener. 
Some elements in the sentence function in conveying 
information which the speaker assumes is present in the 
"consciousness = of the listener \[CHAFE ?fi\]. This 
information is said to be contextually dependent, either 
by virtue of its presence in the preceding discourse or 
because it is part of the shared world knowledge of the 
dialog participants. In a question-answer sys~, 
shared world knowledge refers to information which the 
speaker assumes is present in the database. Information 
functioning in the role just described has been termed 
"given". 
"New" labels all information in the sentence which is 
presented as not retrievable from context. In the 
declarative, elements functioning in asserting 
information What the listener is presumed not to know 
are called new. In the question, elements funci:ioning 
in conveying what the s~eaker wants to know (i.e.- what 
s/he doesn't know) represent information which the 
speaker presumes the listener is not already aware of. 
Flrbas identifies additional functions in the question. 
Of these, (ii) is used here .to aug~mt the 
interpretation of new information. He says: 
"(i) it indicates the want of knowledge on the part 
of the inquirer and appeals to the informant to 
satisfy this want. 
(ii) \[a\] it i,~erts knowledge to the informant in 
that it informs him what the inquirer is 
interested in (what is on her/his mind) and 
* Some other influences on syntactic expression are 
discussed in \[MORGAN and GRE~ 73\]. They surest that 
stylistic reasons, in addition to some of the functions 
discussed here, determine when different syntactic 
constructions are to be used. They point out, for 
example, that the passive tense is often used in 
academic prose to avoid identification of agent and to 
lend a scientific flavor to the text. 
\[b\] from what particular angle the intimated 
want of knowledge is to be satisfied." 
\[FIRBAS 74; \[}.31\] 
Although word order vis-a-vis these and related 
distinctions has been discussed in light of the 
declarative sentence, less has been said about the 
interrogative form. Hellida7 \[HALLII14Y 67\] and 
Krlzkova* are among the few to have analyzed the 
question. Despite the fact that they arrive at 
different conclusions**, the two follow similar lines of 
reasoning. Krlzkova argues that both the wh-item of the 
wh-question and the finite verb (e.g. - "do" or "be') 
of the yes/no question point to the new information to 
be disclosed in the response. These elements she 
claims, ere the only unknowns to the questioner. 
Helllda7, in discussing the yes/no question, also argues 
~at the finite verb is the only unknot. The polarity 
of the text is in question and the finite element 
indicates this. 
In this paper the interpretetion of the unknown elements 
in the question as defined by Krizkova and Helllday is 
followed. The wh-items, in defining the questioner's 
lack of knowledge, act as new information. Firhas' 
analysis of the functions in questions is used to 
further elucidate the role of new information in 
questions. The re~aining elements are given 
information. They represent information assumed by the 
questioner to be true of the database domain. This 
lapeling of information within the question will allow 
the construction of a natural paraphrase, avoiding 
ambiquity. 
V. ~~ 
Following the analysis described above, the CO-OP 
paraphrassr breaks down questions into given and new 
information. ~tore s~ectfically, an input question is 
divided into three parts, of which (2) and (3) form the 
new information. 
(1) given information 
(2) Function ii (a\] from Firhas above 
(3) Function il (b\] from Firhas above 
In terms of the question components, (2) comprises the 
question with no subclauses as it defines the lack of 
knowledge for the hearer. Part (3) comprises the direct 
and indirect modifiers of the interrogative words as 
they indicate the angle from which the question Was 
asked. They define the attributes of the missing 
information for the hearer. Part (1) is fomed from the 
remaining clauses. 
As an exile, consider question (D): 
(D) which division of the computing facility works 
on projects using oceanography research? 
Following the outline above, part (2) of the paraI~rase 
will be the question minus subclauses: ~ich division 
works on proj~-te?', part (3), the modifiers of the 
interrogative words, will be "of the computing facility" 
which modifies =which division'. The remaining clause 
, Summary by (FZRB~ 74\] of the untranslated article 
=The Interrogative Sentence and Some Problems of the 
So-called Functional Sentence Perspective (Contextual 
O~anizatlon of the Sentence\], ~ass rec 4, IS,;8. 
** It ~ould be noted that Halllda 7 and Krizkova discuss 
unknowns in the question in order to define the 
theme end them of a question. Although they agree 
the unkno~ for the questioner, they disagree 
about whlch elements functlon as ~ and whlch 
function as theme. A full discussion of their analysis 
and conclusions is given in \[~XEO~ 79\]. 
68 
"projects using oceanography research" is considered 
given information. The three parts can then be 
assembled into a natural sequence: 
(E) Assuming that there are projects using 
oceanography research, which division works on 
those projects? Look for a division of the 
computing facility.* 
In question (D), information belonging to each of the 
three categories occurred in the question. If one of 
these types of information is missing, the question will 
be presented minus the initial or concluding clauses. 
Only part (2) of the paraphrase will invariably occur. 
If more than one clause occurs in a particular category, 
the question will be furthered splintered. Additional 
given informat ion is parenthesized following the 
"assuming that ..." clause. Example (F) below 
illustrates the paraphrase for a question containing 
several clauses of given information and no clauses 
defining specific attributes of the missing information. 
Clauses containing information characterized by category 
(3) will be presented as separate sentences following 
the stripped-down question. (G) below demonstrates a 
paraphrase containing more than one clause of this type 
of information. 
(F) Q: Which users work on projects in oceanography 
that are sponsored by NASA? 
P: Asst~mlng that there are projects in 
oceanography (those projects are sponsored by 
NASA), which users work on those projects? 
(G) Q: Which programmers in superdlvislon 5000 from 
the ASD group are advised by Thomas Wlrth? 
P: Which programmers are advised by Thomas Wlrth? 
Look for programmers in superdlvlslon 5000. 
The programmers must be from the ~.gD group. 
VI. IMPLEMENTATION OVERVIEW 
The paraphraser's first step in processing is to build a 
tree structure from the representation it is given. The 
tree is then divided into three separate trees 
reflecting the division of given and new information In 
the question. The design of the tree allows for a 
simple set of rules which flatten the tree. The final 
stage of processing in the paraphraser is translation. 
In the translation phase, labels In the parser's 
representation are translated into their corresponding 
words. During this process, necessary transformations 
of the grammar are performed upon the string. 
Several aspects of the implementation will not be 
discussed here, but a description can be found in 
\[MCKEOWN 791. The method used by the paraphraser to 
handle conjunction, disjunction, and limited 
quantification is one of these. A second function of 
the paraphraser is also described In \[MCKEOWN 79\]. The 
set of procedures used to paraphrase the user's query 
can also be used to generate an English version of the 
parser's output. If the tree is not divided into given 
and new information, the flattening and transfor,mtlonal 
rules can be applied to produce a question that is not 
in the three-part form. rn CO-OP, generation is used to 
produce corrections of the user's mistaken 
presupposi tions. 
* This example, as well as all sample questions and 
paraphrases that follow, were, =aken from actual sessions 
with the paraphraser. Question (A)mad its possible 
paraphcases (B) and (C) are the only examples that were 
not run on the paraphraser. 
A. THE PHRA.qE STRUCTURE TREE 
In its initial processing, the paraphraser transforms 
the parser's representation into one that is more 
convenient for generation purposes. The resultant 
structure is a tree that highlights certain syntactic 
features of the question. This initial processing gives 
the paraphraser some independence from the CO-OP system. 
Were the parser's representation changed or the 
component moved to a new system, only the initial 
processing phase need be modified. 
The paraphraser's phrase structure tree uses the main 
verb of the question as the root node of the tree. 1"Ne 
subject of the main verb is the root node of the left 
subtree, the object (if there is one) the root node of 
the right subtree. In the current system, the use of 
binary relations in the parser's representation (see 
\[KAPLAN 79\] for a description of Meta Query Language) 
creates the illusion that every verb or preposition has 
a subject and object. Tne paraphraser's tree does allow 
for the representation of other constructions should the 
incccning language use them. 
Each of the subtrees represents other clauses in the 
question. Both the subject and the object of the main 
verb will have a subtree for each other clause it 
participates in. If a noun in one of these clauses also 
participates in another clause in the sentence, it will 
have subtrees too. 
As an example, consider the question: "~Fnlch active 
users advised by Thomas Wirth work on projects in area 
3?". The phrase structure tree used in the paraphraser 
is shown in Figure I. Since "work" is the main verb, it 
will be the root node of the tree. "users" is root of 
the left subtree, "projects" of the right. Each noun 
participates in one other clause and therefore has one 
subtree. Note that the adjective "active" does not 
appear as part of the tree structure. Instead, it is 
closely bound to the noun it modifies and is treated as 
a property of the noun. 
+7\ 
users projects 
advised by/ ~ in 
Thomas wlrth area 
object object 
Figure i 
B. DIVIDING THE TREE 
Tne constructed tree is computatlonslly suited for the 
three-part paraphrase. The tree is flattened after it 
has been divided into subtrees containing given 
information and the two types of new information. The 
splitting of the tree is accomplished by first 
extracting the topmost smallest portion of the tree 
containing the wh-item. At the very least, this will 
include the root node plus the left and right subtree 
root nodes. This portion of the tree is the stripped 
down question. The clauses ~hlch define the particular 
aspect frora which the question is asked are found by 
searching the left and right subtrees for the wh-ltem or 
questioned noun. The subtree whose root node is the 
wh-item contains these clauses. Note that this may be 
the entire left or right subtree or may only be a 
subtree of one of these. The remainder of the tree 
represents given information. Figure 2 illustrates thls 
division for the previous example. 
69 
i?fo tion 
O: Which acl:ive users advised by Thomas Wtrth work 
on projects in area 3? 
P: Assuming that there are projects in area 3, 
which active users work on those projects? Look 
for users advised by Thomas wirth. 
Figure 2 
C° FLATT~ING 
If the structure of the phrase structure tree is as 
in Figure 3, with A the left subtree and B the 
right, then the following rules define the flattening 
process: 
TREE-> A R B 
SUBTREE -> R' A* B' 
In other words, each of the subtrses will be linearized 
by doing a pre-order treversal of that subtree. As a 
node in a subtres has three pieces of information 
associated with it, one more rule is required to expand 
a node. A node consists of: 
(1) arc-lal~l 
(2) ast-lahel 
(3) subject/object 
where arc-label is the label of the verb or preposition 
used in the parse tree and set-label the label of a noun 
phrase. Subject/object indicates whether the sub-node 
noun phrase functions as subject or object in the 
clause; it is used by the subject-aux transformation and 
does not apply to the expansion rule. The following 
rule expands a node: 
NODE -> ARC-tABEL SET-LABEL 
TWo transformations are applied during the flattening 
process. They are wh-frontlng and subject-aux 
inversion. They are further described in the section on 
transformations. 
Tree: a Subtree: 
B' 
Figure 3 
The tree of given information is flattened first. It is 
part of the left or right subtree of the phrase 
structure tree and therefore is flattened by a pre-order 
traversal. It is during the flattening stage that the 
words "Assuming that there \[be\] ... • are inserted to 
introduce the clause of given information. "Be" will 
agree with the subject of the clause. If there is more 
than one clause, parentheses are inserted around the 
additional ones. The tree representing the stripped 
doom question is flattened next. It is followed by the 
modifiers of the questioned no~1. The phrase "Look for" 
is inserted before the first clause of modifiers. 
70 
D. TRANSFORMATIONS 
The graewar used in the paraphraser is a 
transformational one. In addition to the basic 
flattening rules described above, the following 
transformations are used: 
~an~ -fr°nting 
ation 
~.do-support 
(~subject-aux inversion 
~f flx-hopping 
kcontrsction 
has deletion 
The curved lines indicate the ordering restrictions. 
There are two connected groups of transformations. If 
wh-fronting applies, then so will do-support, 
subJect-aux inversion, and affix-hopplng. The second 
group of transformations is invoked through the 
application of negation. It includes do-support, 
contraction, and affix-hopping. Has-deletion is not 
affected b 7 the absence or presence of other 
tranafomations. A description of the transformation 
rules follo~. The rules used here are based on 
analyses described by \[~IAN and ~ 75\] and 
analyses described by \[CULLICOV~ 76\]. 
The rule for wh-fronting is specified as follows, where 
SD abbreviates structural description and SC, structural 
change: 
SD: X - NP - Y 
i 2 3 
SC: 2+i 0 3 
condition: 2 dominates wh 
The first step in the implementation of wh-fronting is a 
search of the tree for the wh-item. A slightly 
different approach is used for paraphrasing than is used 
for generation. The difference occurs because in the 
original question, the NP to be fronted may be the head 
noun of some relative clauses or prepositional phrases. 
When generating, these clauses must be fronted along 
with the heed noun. Since the clauses of the original 
que~ion are broken down for the paraphrase, it will 
never he the case when pars~hrssing that the NP to be 
fronted also dominates relative clauses or prepositional 
phrases. For this reason, when paraphrase mode is used, 
the applicability of wh-fronting is tasted for and is 
applied in the flattening process of the stripped down 
question. If it applies, only one word need be moved to 
the initial position. 
When generation is being done, the applicability of 
wh-fronting is tested for immediately before flattening. 
If the transformation applies, the tree is split. The 
subtree of which the wh-itmn is the root is flattened 
separstely from the remair~er of the tree and is 
attached in fronted position to the string resulting 
from flattening the other part. 
After wh-fronting has been appl led, do-support is 
invoked. In CO-OP, the underlying representation of the 
q~aation does not contain mudals or auxiliary verbs. 
Thus, fronting the wh-item necessitates supplying an 
auxiliary. The following rule is used for do-support: 
SD: NP - NP - tense - V - X 
1 2 3 4 
SC= 1 do+2 3 4 
condition= 1 dominates wh 
SubJect-aux inversion is activated immediately 
afterwards. Aqaln, if wh-frontlng applied, subject-aux 
inversion will apply also. The rule is= 
SD: NP - NP - AUX - X 
I 2 3 4 
SC: I 3+2 0 4 
condition: i dominates wh 
Affix-hopping follows subject-aux inversion. In the 
Paraphraser it is a combination of what is commonly 
thought of as afflx-hopplng and number-agreement. Tense 
and number are attributes of all verbs in the Parser's 
representation. When an auxiliary is generated, the 
tense and n~nber are "hopped" from the verb to the 
auxiliary. Formally: 
SD: X - AUX - Y - tense-nua~-V - Z 
i 2 3 4 5 6 
SC: 1 2+4 3 0 5 6 
Some transformational analyses propose that wh-frontlng 
and subJect-aux inversion aPPly to the relative clause 
as well as the question. In the CO-OP Paraphraser, the 
heed-noun is properly positioned by the flattening 
process and wh-frontlng need not be used. Subject-aux 
inversion however, may be applicable. In cases where 
the head noun of the clause is not its subject, 
subject-aux inversion results in the proper order. 
• The rule for negation is tested during the translation 
phase of execution. It has been formalized as: 
SD: X - tense-V - NP - Y 
1 2 3 4 
SC: i 2+no 3 4 
condition: 3 marked as negative 
In Ehe CO-OP representation, an indication of negation 
is carried on the object of a binary relation (see 
\[KAPLAN 79\] ). When generating an English representation 
of the question, it is possible in some cases to express 
negation as modification of the noun (see question (H) 
below). In all cases however, negation can be indicated 
as Part of the verb (see version (I) of question (H)). 
Therefore, when the object is marked as negative, the 
Paraphraser moves the negation to heroine Part of the 
verbal element. 
(R) which students have no advisors? 
(I) Which students don't have advisors? 
In English, the negative marker is attached to the 
auxiliary of the verbal element and therefore, as was 
the case for questions, an auxiliary must be generated. 
Do-support is used. The rule used for do-support after 
negation differs from the one used after wh-frontlng. 
They are presented this way for clarity, but could have 
been combined into one rule. 
SD: X - tense-V-no - Y 
1 2 3 
SC: 1 do+2 3 
Affix-hopping, as described above, hops the tense, 
number, and negation from the verb to the auxiliary 
verb. The cycle of transformations invoked thru 
application of negation is completed with the 
contraction transformation. The statement of the 
contraction transformation Is" 
SD: X - do+tense -no - Y 
1 2 3 4 
SC: I #2+n* t# 0 4 
where # indicates that the result must he treated as a 
unit for further transformations. 
VII. CONCLUSIONS 
The paraphraser described here is a sylltactic one. 
while this work has examined the reasons for different 
forme .)f expression, additions must be made in the area 
of semantics. The substitution of synonyms, phrases, or 
idioms for portions or all of the question requires an 
examination of the effect of context on word meaning and 
of the intentions of the speaker on word or phrase 
choice. The lack of a rich semantic base and contextual 
information dictated the syntactic approach used here, 
but the paraphraser can be extended once a wider range 
of information becomes available. 
The CO-OP paraphraser has been designed to be 
domain-independent and thus a change of the database 
requires no charges in the paraphraser. Paraphrasers 
which use the template form hbwever, will require such 
changes. This is because the templates or patterns, 
which constitute the type of question that can be asked, 
are necessarily dependent on the domain. For different 
databases, a different set of templates must be used. 
The CO-OP Paraphraser also differs from other systems in 
that it generates the question using a transformational 
grammar of questions. It addresses two specific 
problems involved in generating paraphrases-" 
I. ambiguity in determining which noun phrases a 
relative clause modifies 
2. the production of a question that differs from 
the user' s 
These goals have been achieved for questions using 
relative clauses through the application of a theory of 
given and new information to the generation process. 
~E~NTS 
Thls work was partially supported by an IBM fellowship 
and NSF grant MCS78-08401. I would like to thank Dr. 
Aravind K. Joshi and Dr. Bonnie Webbar for their 
invaluable comments on the style and content of this 
paper. 
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