Natural language information retrieval:progress report
School of Communication,Information and Library Studies,Rutgers University,New Brunswick,NJ 04612,USA
GE Corporate Research and Development,Niskayuna,NY 12309,USA
Natural language processing (NLP) techniques may hold a tremendous potential for overcoming the
inadequacies of purely quantitative methods of text information retrieval,but the empirical evidence to
support such predictions has thus far been inadequate and appropriate scale evaluations have been slow
to emerge.In this paper,we report on the progress of the Natural Language Information Retrieval
project,a joint eort of several sites led by GE Research and its evaluation the 6th Text Retrieval
Conference (TREC-6).In this paper we describe the`stream architecture',a method we designed to
combine evidence obtained from several dierent document representations.Some of the document
representations used in the experiments described here involved the use of phrases and proper names
computed using Natural Language Processing techniques.#1999 Elsevier Science Ltd.All rights
1.Introduction and motivation
Recently,we noted a renewed interest in using NLP techniques in information retrieval,
sparked in part by the sudden prominence,as well as the perceived limitations,of existing IR
technology in rapidly emerging commercial applications,including on the Internet.This has
also been re¯ected in what is being done at TREC:using phrasal terms and proper name
annotations became a norm among TREC participants and a special interest track on NLP
took o for the ®rst time in TREC-5.
In this paper we discuss particulars of the joint GE/Rutgers TREC-6 entry.
The main thrust of this project has been to demonstrate that robust if relatively shallow
Information Processing and Management 36 (2000) 155±178
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PII:S0306-4573( 99)00049- 7
* Corresponding author.
E-mail address:firstname.lastname@example.org (J.Perez-Carballo).
NLP can help to derive better representation of text documents for indexing and search
purposes than any simple word and string-based methods commonly used in statistical full-text
retrieval.This was based on the premise that linguistic processing can uncover certain critical
semantic aspects of document content,something that simple word counting cannot do,thus
leading to more accurate representation.The project's progress has been rigorously evaluated
in a series of ®ve Text Retrieval Conferences (TRECs) organized by the US Government under
the guidance of NIST and DARPA.Since 1995,the project scope widened substantially to
include several parallel eorts at GE,Rutgers,Lockheed Martin Corporation,New York
University,University of Helsinki and Swedish Institute for Computer Science (SICS).We
have also collaborated extensively with SRI International.At TREC we demonstrated that
NLP can be done eciently on a very large scale and that it can have a signi®cant impact on
IR.At the same time,it became clear that exploiting the full potential of linguistic processing
is harder than originally anticipated.Our initial eort was directed at using NLP techniques to
extract meaningful indexing terms and to assist a statistical information retrieval system in
making a proper use of them.To that end we had to rethink how NLP was to be done when
instead of a couple of megabytes of CACM abstracts
we faced thousands of megabytes of
newspaper stories,patent disclosure statements,and government documents.By TREC-3
(1994),we were able to parse and otherwise process gigabytes of free text and indeed show the
gains in retrieval eectiveness against our own non-NLP baseline.However,the results
remained inconclusive as the baseline which we used as an initial benchmark turned out to be
signi®cantly lower than that of the leading statistical IR systems.The main achievement of
these early TREC experiments was therefore their very scale,which showed that NLP,in
however tentative form,was now available for serious IR research.By TREC-4 (1995),we
were ®nally ready to turn our attention to the performance issues.
Not surprisingly,we have soon noticed that the amount of improvement in recall and
precision which we could attribute to NLP,appeared to be related to the quality of the initial
search request,which in turn seemed unmistakably related to its length (cf.Table 1,where T-2,
T-3,T-4 refer respectively to TRECs 2,3 and 4).Long and descriptive queries responded well
to NLP,while terse one-sentence search directives showed hardly any improvement.This was
not particularly surprising or even new,considering that the shorter queries tended to contain
Performance gains attributed to NLP indexing vs.query length (T-2,T-3,T-4 refer respectively to TRECs 2,3
T-2:0115 terms T-3:070 terms T-4:010 terms
Runs base +NL base +NL Base +NL
Prec.0.22 0.31 0.22 0.27 0.20 0.22
Change +40% +20% +10%
CACM-3204 is a collection of technical abstracts from the Communications of the ACM journal,about 2
Mbytes,including 50 or so queries.It was used as one of the standard test collections prior to TREC.
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178156
highly discriminating words in them and that was just enough to achieve the optimal
performance.On the other hand,comparing various evaluation categories at TREC,it was
also quite clear that the longer queries just did better than the short ones,no matter what their
level of processing.Furthermore,while the short queries needed no better indexing than with
simple words,their performance remained inadequate and one de®nitely could use better
queries.Therefore,we started looking into ways to build full-bodied search queries,either
automatically or interactively,out of users'initial search statements.This is a challenging
undertaking,one that promises to move NLP-IR relationship into a more advanced next level.
The fact that NLP-based indexing is evidently able to further accelerate the gains resulting
from using better queries is also worth exploring.
TREC-5 (1996),therefore,marks a shift in our approach away from text representation
issues and towards query development problems.While our TREC-5 system still performs
extensive text processing in order to extract phrasal and other indexing terms,our main focus
moved on to query construction using words,sentences,and entire passages to expand initial
search speci®cations in an attempt to cover their various angles,aspects and contexts.Based on
the observations that NLP is more eective with highly descriptive queries,we designed an
expansion method in which entire passages from related,though not necessarily relevant
documents were quite liberally imported into the user queries.This method appeared to have
produced a dramatic improvement in the performance of two dierent statistical search engines
that we tested in our TREC-5 experiments (Cornell's SMART and NIST's Prise) boosting the
average precision by anywhere from 40% to as much as 130%.Similar improvements were
reported for the University of Massachusetts'INQUERY system,when run on our expanded
queries (personal communication with J.Callan).Therefore,topic expansion appears to lead to
a genuine,sustainable advance in IR eectiveness.Moreover,we show in TREC-6 and TREC-
7 that this process can be automated while maintaining the performance gains.
The other notable new feature of our TREC-5 system is the stream architecture.It is a
system of parallel indexes built for a given collection,with each index re¯ecting a dierent text
representation strategy.These indexes are called streams because they represents dierent
streams of data derived from the underlying text archive.A retrieval process searches all or
some of the streams and the ®nal ranking is obtained by merging individual stream search
results.This allows for an eective combination of alternative document representation and
retrieval strategies,in particular various NLP and non-NLP methods.The resulting meta-
search system can be optimized by maximizing the contribution of each stream.It is also a
convenient vehicle for an objective evaluation of streams against one another.
1.1.NLP-based indexing in information retrieval
In information retrieval (IR),a typical task is to fetch relevant documents from a large
archive in response to a user's query and rank these documents according to relevance.This
has been usually accomplished using statistical methods (often coupled with manual encoding)
that (a) select terms (words,phrases,and other units) from documents that are deemed to best
represent their content,and (b) create an inverted index ®le (or ®les) that provide an easy
access to documents containing these terms.A subsequent search process will attempt to match
preprocessed user queries against term-based representations of documents in each case
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178 157
determining a degree of relevance between the two which depends upon the number and types
of matching terms.Although many sophisticated search and matching methods are available,
the fundamental problem remains to be an adequate representation of content for both the
documents and the queries.
In term-based representation,a document (as well as a query) is transformed into a
collection of weighted terms (or surrogates representing combinations of terms),derived
directly from the document text or indirectly through thesauri or domain maps.The
representation is anchored on these terms and thus their careful selection is critical.Since each
unique term can be thought to add a new dimensionality to the representation,it is equally
critical to weigh them properly against one another so that the document is placed at the
correct position in the N-dimensional term space
.Our goal is to have the documents on the
same topic placed close together,while those on dierent topics placed suciently apart.The
above should hold for any topics,a daunting task indeed,which is additionally complicated by
the fact that we often do not know how to compute terms weights.The statistical weighting
formulas,based on terms distribution within the database,such as tfidf,are far from optimal
and the assumptions of term independence which are routinely made are false in most cases.
This situation is even worse when single-word terms are intermixed with phrasal terms and the
term independence becomes harder to justify.
The simplest word-based representations of content,while relatively better understood,are
usually inadequate since single words are rarely speci®c enough for accurate discrimination
and their grouping is often accidental.A better method,it may seem,is to identify groups of
words that create meaningful phrases,especially if these phrases denote important concepts
in the database domain.For example,``joint venture''is an important term in the Wall
Street Journal (WSJ henceforth) database,while neither``joint''nor``venture''are important
by themselves.In our early retrieval experiments with the TREC database,we noticed that
both``joint''and``venture''were dropped from the list of terms by the system (NIST's
PRISE,see Strzalkowski and Perez-Carballo,1994) because their inverted document
frequency (idf) weights were too low.In large databases,it is quite common to eliminate
very high frequency terms to conserve space,because of their minimal discriminating value.
On the other hand,a phrase,even one made up of high frequency words may make a good
discriminator,therefore,the use of phrasal terms becomes not merely desirable,but in fact
necessary.This observation has been made by a growing number of IR researchers and
practitioners,for example,many systems participating in TREC now use one or another
form of phrase extraction.
There are a number of ways to obtain`phrases'from text.These include generating simple
collocations,statistically validated N-grams,part-of-speech tagged sequences,syntactic
structures and even semantic concepts.Some of these techniques are aimed primarily at
identifying multi-word terms that have come to function like ordinary words,for example
``white collar''or``electric car''and capturing other co-occurrence idiosyncrasies associated
with certain types of texts.This simple approach has proven quite eective for some systems,
In a vector-space model term weights are represented as coordinate values;in a probabilistic model estimates of
prior probabilities are used.
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for example the Cornell group reported (Buckley,Singhal,Mitra & Salton,1995) that adding
simple collocations to the list of available terms can increase retrieval precision by as much
as 10%.More recently (Mitra,Buckley,Singhal & Cardie,1997) Cornell's improvement due
to phrases went down from about 7% before they introduced a new weighting scheme
(lnultu) to less than 1%.Experiments in this paper report about 3% improvement from
Other more advanced techniques of phrase extraction,including extended N-grams and
syntactic parsing,attempt to uncover`concepts',which would capture underlying semantic
uniformity across various surface forms of expression.Syntactic phrases,for example,appear
reasonable indicators of content,arguably better than proximity-based phrases,since they
can adequately deal with word order changes and other structural variations (e.g.,``college
junior''vs.``junior in college''vs.``junior college'').A subsequent regularization process,
where alternative structures are reduced to a``normal form'',helps to achieve the desired
uniformity,for example,``college+junior''will represent a college for juniors,while
``junior+college''will represent a junior in a college.A more radical normalization would
have also`verb object',`noun rel-clause',etc.converted into collections of such ordered
pairs.This head+modi®er normalization has been used in our system and is further
described in this chapter.In order to obtain the head+modi®er pairs of respectable quality,
we used a full-scale robust syntactic parsing (TTP,see Section 3.1.3) In 1998,in
collaboration with the University of Helsinki,we used their Functional Dependency
Grammar system to perform all linguistic analysis of TREC data and to derive multiple
dependency-based indexing streams.
Although the results we have obtained using syntactic analysis of the text have shown some
improvements we still have not obtained the dramatic results that would indicate a break
through.One possible explanation is that the syntactic analysis is just not going far enough.Or
perhaps more appropriately,that the semantic uniformity predictions made on the basis of
syntactic structures (as is in the case of head+modi®er pairs) are less reliable than we have
hoped for.Of course,the relatively low quality of parsing may be a major problem,although
there is little evidence to support that.In other words,if we are shooting for some``semantic
concept-based representation'',are there other ways to get there?
This state of aairs has prompted us to take a closer look at the phrase selection and
representation process.In TREC-3 we showed that an improved weighting scheme for
compound terms,including phrases and proper names,leads to an overall gain in retrieval
accuracy.The fundamental problem,however,remained to be the system's inability to
recognize,in the documents searched,the presence or absence of the concepts or topics that
the query was asking for.The main reason for this was,we noted,the limited amount of
information that the queries could convey on various aspects of topics they represented.
Therefore,we started experimenting with manual and automatic query building techniques.
The purpose of this exercise was to devise a method for full-text query expansion that would
allow for creating fuller search queries such that:(1) the performance of any system using these
queries would be signi®cantly better than when the system is run using the original queries and
(2) the method could be eventually automated or semi-automated so as to be useful to a
nonexpert user.Our preliminary results from TREC-5 evaluations show that this approach is
indeed very eective.
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1.2.NLP in information retrieval:a perspective
Natural language processing has always seemed to oer the key to building the ultimate
information retrieval system.Somehow we feel that the`bag-of-words'representations,
prevalent among today's information retrieval systems,can hardly do justice to the
complexities of free,unprocessed text with which we have to deal.Some of the favorite
examples include Venetian blind vs.blind Venetian,Poland is attacked by Germany vs.Germany
attacks Poland,or car wash vs.automobile detailing.Natural language processing could provide
solutions to at least some of these problems through lexical and syntactic analysis of text (e.g.,
Venetian is used as either an adjective or a noun),through the assignment of logical structures
to sentences (e.g.,Germany is a logical subject of attack),or through an advanced semantic
analysis that may involve domain knowledge.Other important applications include discourse-
level processing,resolution of anaphoric references,proper name identi®cation and more.
Unfortunately,a direct application of NLP techniques to information retrieval has met some
rather severe obstacles,chief among which was a paralyzing lack of robustness and eciency.
Worse yet,the diculties did not end with the linguistic processing itself but extended to the
representation it produced:it was not at all clear how the complex structures could be
eectively compared to determine relevance.A better approach,it seemed,was to use NLP to
assist an IR system,whether Boolean,statistical,or probabilistic,in automatically selecting
important terms,words and phrases,which could then be used in representing documents for
search purposes.This approach provided extra manoeuvrability for softening any inadequacies
of the NLP software without incapacitating the entire system.Eciency problems still
prevented direct on-line processing of any large amount of text,but NLP could be gradually
built into an o-line database indexing.
A common theme among the majority of NLP applications to information retrieval is to use
linguistically motivated objects (stems,phrases,proper names,®xed terminology,lexical
correlations,etc.) as derived from documents and queries,and create a`value-added'
representation,usually in the form of an inverted index.The bulk of this representation may
still rest upon statistically weighted single-word terms,while additional terms (e.g.,phrases) are
included on the assumption that they can only make the representation richer,and in eect,
improve the eectiveness of subsequent search.For example,if the search ®nds the phrase
Venetian blind in a document,we should have more con®dence as to the relevance of this
document than when Venetian and blind are found separately.This,however,does not seem to
happen in any consistent manner.The problem is that the phrases are not all alike and their
abilities to re¯ect the content of the text vary greatly with the type of the phrase and its
position within the text.Statistical weighting schemes developed for single-word terms,such as
tfidf,do not seem to extend on compound terms.Moreover,compound terms derived through
statistical means (e.g.,co-occurrence,mutual information) tend to behave dierently from
those derived with a grammar-based parser.A retrieval model,which includes a weighting
scheme for terms,is a crucial part of any information retrieval system and a wrong retrieval
model can defeat even the most accomplished representation.Nonetheless,most work on NLP
in IR concentrated on representation or compound term matching strategies,with relatively
little consideration given to term weighting and to scoring of retrieved documents.Some
commonly used strategies,where a phrase weight was a function of weights of its components,
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did not produce uniform results,(Fagan,1987;Lewis & Croft,1990).In fact,the lack of an
established retrieval model which can handle linguistically-motivated compound terms may be
one of the more serious obstacles in evaluating the impact and feasibility of natural language
processing in information retrieval.
In recent years,we noted a renewed interest in using NLP techniques in information
retrieval,sparked in part by the sudden prominence,as well as the perceived limitations,of
existing IR technology in rapidly emerging commercial applications,including on the Internet.
This has also been re¯ected in what is being done at TREC:using phrasal terms and proper
name annotations became a norm among TREC participants and a special interest track on
NLP took o for the ®rst time in TREC-5.
In the remainder of this paper we discuss particulars of our present system and some of the
observations made while processing TREC data.The above comments will provide the
background for situating our present eort and the state-of-the-art with respect to where we
should be in the future.
2.Stream-based Information Retrieval Model
The stream model was conceived to facilitate a thorough evaluation and optimization of
various text content representation methods,including simple quantitative techniques as well as
those requiring complex linguistic processing.Our system encompasses a number of statistical
and natural language processing techniques that capture dierent aspects of document content:
combining these into a coherent whole was in itself a major challenge.Therefore,we designed
a distributed representation model in which alternative methods of document indexing (which
we call`streams') are strung together to perform in parallel.Streams are built using a mixture
of dierent indexing approaches,term extracting and weighting strategies,even dierent search
The following term extraction steps correspond to some of the streams used in our system:
1.Elimination of stopwords:Original text words minus certain no-content and low-content
stopwords are used to index documents.Included in the stopwords category are closed-class
words such as determiners,prepositions,pronouns,etc.,as well as certain very frequent
2.Morphological stemming:Words are normalized across morphological variants (e.g.,
``proliferation'',``proliferate'',``proliferating'') using a lexicon-based stemmer.This is done
by chopping o a sux (-ing,-s,-ment ) or by mapping onto root form in a lexicon (e.g.,
proliferation to proliferate).
3.Phrase extraction:Various shallow text processing techniques,such as part-of-speech
tagging,phrase boundary detection and word co-occurrence metrics are used to identify
relatively stable groups of words,e.g.,joint venture.
4.Phrase normalization:``Head+Modi®er''pairs are identi®ed in order to normalize across
syntactic variants such as weapon proliferation,proliferation of weapons,proliferate weapons,
etc.,and reduce to a common`concept',e.g.,weapon+proliferate.
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5.Proper name extraction:Proper names are identi®ed for indexing,including people names
and titles,location names,organization names,etc.
The ®nal results are produced by merging ranked lists of documents obtained from searching
all streams with appropriately preprocessed queries,i.e.,phrases for phrase stream,names for
names stream,etc.The merging process weights contributions from each stream using a
combination that was found the most eective in training runs.This allows for an easy
combination of alternative retrieval and routing methods,creating a meta-search strategy
which maximizes the contribution of each stream.Cornell's SMART (Salton,1989) and
Umass'Inquery (Broglio,Callan & Croft,1994.See also:http://ciir.cs.umass.edu/
demonstrations/InQueryRetrievalEngine.shtml) information retrieval systems were used as
search engines for dierent streams.
Among the advantages of the stream architecture we may include the following:
.stream organization makes it easier to compare the contributions of dierent indexing
features or representations.For example,it is easier to design experiments which allow us to
decide if a certain representation adds information which is not contributed by other
.it provides a convenient testbed to experiment with algorithms designed to merge the results
obtained using dierent IR engines and/or techniques.
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.it becomes easier to ®ne-tune the system in order to obtain optimum performance
.it allows us to use any combination of IR engines without having to adapt them in any way.
Figure 1 shows an example in which the raw text from the Text Database is processed by 4
dierent ways in order to produce corresponding indexes.The query is processed in the same
way to produce a representation appropriate for each index.A merging process takes the
ranked sets resulting from matching the appropriate form of the query with the appropriate
index and merges them into a ®nal single ranked set.
The notion of combining evidence from multiple sources is not new in information retrieval.
Several researchers have noticed in the past that dierent systems may have similar
performance but retrieve dierent documents,thus suggesting that they may complement one
another.It has been reported that the use of dierent sources of evidence increases the
performance of a hybrid system (see for example:Callan,1995;Fox,1993;Saracevic &
Kantor,1988).Nonetheless,the stream model used in our system is unique in that it explicitly
addresses the issue of document representation as well as provides means for subsequent
3.Advanced linguistic streams
3.1.Head+modi®er pairs stream
Our linguistically most advanced stream is the head+modi®er pairs stream.In this stream,
documents are reduced to collections of word pairs derived via syntactic analysis of text
followed by a normalization process intended to capture semantic uniformity across a variety
of surface forms,e.g.,``information retrieval'',``retrieval of information'',``retrieve more
information'',``information that is retrieved'',etc.are all reduced to the pair
``retrieve+information'',where``retrieve''is a head or operator and``information''is a
modi®er or argument.It has to be noted that while the head-modi®er relation may suggest
semantic dependence,what we obtain here is strictly syntactic,even though the semantic
relation is what we are really after.This means in particular that the inferences of the kind
where a head+modi®er is taken as a specialized instance of head,are inherently risky,because
the head is not necessarily a semantic head,and the modi®er is not necessarily a semantic
modi®er,and in fact the opposite may be the case.In the experiments that we describe here,
we have generally refrained from semantic interpretation of head-modi®er relationship,treating
it primarily as an ordered relation between otherwise equal elements.Nonetheless,even this
simpli®ed relationship has already allowed us to cut through a variety of surface forms and
achieve what we thought was a nontrivial level of normalization.By``normalization''we mean
that the same head-modi®er term represents a pair of words that may appear in the text in
dierent order,with dierent suxes or with other words in between.The apparent lack of
success of linguistically-motivated indexing in information retrieval may suggest that we
haven't still gone far enough.
In our system,the head+modi®er pairs stream is derived through a sequence of processing
steps that include:
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2.Lexicon-based word normalization (extended`stemming').
3.Syntactic analysis with TTP parser.
4.Extraction of head+modi®er pairs.
5.Corpus-based disambiguation of long noun phrases.
These steps are described below.For more details the reader is referred to TREC reports,
and other works,including (TREC-6:Strzalkowski et al.,1997,TREC-5:Strzalkowski et al.,
Part of speech tagging allows for resolution of lexical ambiguities in a running text,
assuming a known general type of text (e.g.,newspaper,technical documentation,medical
diagnosis,etc.) and a context in which a word is used.This in turn leads to a more accurate
lexical normalization or stemming.It also is a basis for a phrase boundary detection.
We used a version of Brill's rule based tagger (Brill,1992) trained on Wall Street Journal
texts to preprocess linguistic streams used by SMART.This system is based on the Penn
Treebank Tagset developed at the University of Pennsylvania and have compatible levels of
3.1.2.Lexicon-based word normalization
Word stemming has been an eective way of improving document recall since it reduces
words to their common morphological root,thus allowing more successful matches.On the
other hand,stemming tends to decrease retrieval precision,if care is not taken to prevent
situations where otherwise unrelated words are reduced to the same stem.In our system we
replaced a traditional morphological stemmer with a conservative dictionary-assisted sux
The sux trimmer performs essentially two tasks:
1.it reduces in¯ected word forms to their root forms as speci®ed in the dictionary,and
2.it converts nominalized verb forms (e.g.,``implementation'',``storage'') to the root forms of
corresponding verbs (i.e.,``implement'',``store'').
This is accomplished by removing a standard sux,e.g.,``stor+age'',replacing it with a
standard root ending (``+e''),and checking the newly created word against the dictionary,i.e.,
we check whether the new root (``store'') is indeed a legal word.
3.1.3.Syntactic analysis with TTP
Parsing reveals ®ner syntactic relationships between words and phrases in a sentence,
relationships that are hard to determine accurately without a comprehensive grammar.Some of
these relationships do convey semantic dependencies,e.g.,in Poland is attacked by Germany the
subject+verb and verb+object relationships uniquely capture the semantic relationship of who
Dealing with pre®xes is a more complicated matter,since they may have quite strong eect upon the meaning of
the resulting term,e.g.,`un-'usually introduces explicit negations.
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attacked whom.The surface word-order alone cannot be relied on to determine which
relationship holds.From the onset,we assumed that capturing semantic dependencies may be
critical for accurate text indexing.One way to approach this is to exploit the syntactic
structures produced by a fairly comprehensive parser.
The TTP (Tagged Text Parser) was designed and built by one of authors (TS) and it is based
on the Linguistic String Grammar developed by Sager (1981).The parser currently
encompasses some 400 grammar productions,but it is by no means complete.The parser's
output is a regularized parse tree representation of each sentence,that is,a representation that
re¯ects the sentence's logical predicate-argument structure.For example,logical subject and
logical object are identi®ed in both passive and active sentences and noun phrases are
organized around their head elements.The parser is equipped with a powerful skip-and-®t
recovery mechanism that allows it to operate eectively in the face of ill-formed input or under
a severe time pressure.TTP has been shown to produce parse structures which are no worse
than those generated by full-scale linguistic parsers when compared to hand-coded Treebank
parse trees (Strzalkowski and Scheyen,1996).
3.1.4.Extracting head+modi®er pairs
Syntactic phrases extracted from TTP parse trees are head+modi®er pairs.The head in
such a pair is a central element of a phrase (main verb,main noun,etc.),while the
modi®er is one of the adjunct arguments of the head.It should be noted that the
parser's output is a predicate-argument structure centered around main elements of various
The following types of pairs are considered:(1) a head noun and its left adjective or
noun adjunct,(2) a head noun and the head of its right adjunct,(3) the main verb of a
clause and the head of its object phrase and (4) the head of the subject phrase and the
main verb.These types of pairs account for most of the syntactic variants for relating two
words (or simple phrases) into pairs carrying compatible semantic content.This also gives
the pair-based representation sucient ¯exibility to eectively capture content elements
even in complex expressions.There are of course exceptions.For example,the three word
phrase``former Soviet president''would be broken into two pairs``former president''and
``Soviet president'',both of which denote things that are potentially quite dierent from
what the original phrase refers to and this fact may have potentially a negative eect on
retrieval precision.This is one place where a longer phrase appears more appropriate.
Below is a small sample of head+modi®er pairs extracted (proper names are not
While serving in South Vietnam,a number of US Soldiers were reported
as having been exposed to the defoliant Agent Orange.The issue is
veterans entitlement,or the awarding of monetary compensation and/or
medical assistance for physical damages caused by Agent Orange.
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3.1.5.Corpus-based disambiguation of long noun phrases
The phrase decomposition procedure is performed after the ®rst phrase extraction pass in
which all unambiguous pairs (noun+noun and noun+adjective) and all ambiguous noun
phrases are extracted.Any nominal string consisting of three or more words of which at
least two are nouns is deemed structurally ambiguous.In the TREC corpus,about 80% of
all ambiguous nominals were of length 3 (usually 2 nouns and an adjective),19% were of
length 4 and only 1% were of length 5 or more.The phrase decomposition algorithm has
been described in detail in (Strzalkowski,1995).The algorithm was shown to provide about
70% recall and 90% precision in extracting correct head+modi®er pairs from 3 or more
word noun groups in TREC collection texts.In terms of the total number of pairs
extracted unambiguously from the parsed text,the disambiguation step recovers an
additional 10 to 15% of pairs,all of which would otherwise be either discarded or
3.2.Simple noun phrase stream
In contrast to the elaborate process of generating the head+modi®er pairs,unnormalized
noun groups are collected from part-of-speech tagged text using a few regular expression
patterns.No attempt is made to disambiguate,normalize,or get at the internal structure of
these phrases,other than the stemming which has been applied to text prior to the phrase
extraction step.The following phrase patterns have been used,with phrase length arbitrarily
limited to the maximum 7 words:
1.a sequence of modi®ers (adjectives,participles,etc.) followed by at least one noun,such as:
``cryonic suspension'',``air trac control system'';
2.proper noun sequences modifying a noun,such as:``us citizen'',``china trade'';
3.proper noun sequences (possibly containing``&''):`warren commission'',``national air trac
The motivation for having a phrase stream is similar to that for head+modi®er pairs since
both streams attempt to capture signi®cant multi-word indexing terms.The main dierence is
the lack of normalization,which makes the comparison between these two streams particularly
In our system names are identi®ed by the parser and then represented as strings,e.g.,
south+africa.The name recognition procedure is extremely simple,in fact little more than the
scanning of successive words labeled as proper names by the tagger (``np''and``nps''tags).
Single-word names are processed just like ordinary words,except for the stemming which is
not applied to them.We also made no eort to assign names to categories,e.g.,people,
companies,places,etc.,a classi®cation which is useful for certain types of queries (e.g.,``To be
relevant a document must identify a speci®c generic drug company'').In the TREC-5 database,
compound names make up about 8% of all terms generated.A small sample of compound
names extracted is listed below:
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The stems stream is the simplest,yet the most eective of all streams,a backbone of the
multi-stream model.It consists of stemmed single-word tokens (plus hyphenated phrases) taken
directly from the document text (exclusive of stopwords).The stems stream provides the most
comprehensive,though not very accurate,image of the text it represents and therefore it is able
to outperform other streams that we used thus far.We believe however,that this
representation model has reached its limits and that further improvement can only be achieved
in combination with other text representation methods.This appears consistent with the results
reported at TREC.
In addition,we use WordNet (Fellbaum,1998) to identify unambiguous single-sense words
and give them premium weights as reliable discriminators.Many words,when considered out
of context,display more than one sense in which they can be used.When such words are used
in text they may assume any of their possible senses thus leading to undesired matches.This
has been a problem for word based IR systems and have spurred attempts at sense
disambiguation in text indexing (Krovetz and Croft,1992).Another way to address this
problem is to focus on words that do not have multiple-sense ambiguities and treat these as
special,because they seem to be more reliable as content indicators.This modi®cation has
produced a slightly stronger stream.
The results in Table 2 are somewhat counter-intuitive,particularly the unexpectedly weak
performance of H+M Pairs stream.While we have noticed that Phrases often outperform
Pairs (see for example TREC-5 paper:Strzalkowski et al.1996),the dierence was never this
pronounced.One possible explanation is a worse than expected quality of parse structures
generated by TTP,which may be related to suboptimal setting of critical parameters,
particularly the time-out value.We continue to investigate these results.
For streams using SMART indexing,we selected optimal term weighting schemes from
How dierent streams perform relative to one another (11-pt avg.Prec) (TREC 6 data)
short queries long queries
Stems 0.1070 0.2684
Phrases 0.0846 0.2541
H+M Pairs 0.0405 0.1787
Names 0.0648 0.0753
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among a dozen or so variants implemented with version 11 of the system (see Table 3).These
schemes vary in the way they calculate and normalize basic term weights.For example,in
lnc.ntn scheme,lnc scoring (log-tf,no-idf,cosine-normalization) is applied to documents,and
ntn scoring (straight-tf,idf,no-normalization) is applied to query terms.The selection of one
scheme over another can have a dramatic eect on system's performance.For details the
reader is referred to Buckley,1993.
4.Stream merging and weighting
The results obtained from dierent streams are lists of documents ranked in order of
relevance:the higher the rank of a retrieved document,the more relevant it is presumed to be.
In order to obtain the ®nal retrieval result,ranking lists obtained from each stream have to be
combined together by a process known as merging or fusion.The ®nal ranking is derived by
calculating the combined relevance scores for all retrieved documents.The following are the
primary factors aecting this process:
1.document relevancy scores from each stream
2.retrieval precision distribution estimates within ranks from various streams,e.g.,projected
precision between ranks 10 and 20,etc.,
3.the overall eectiveness of each stream (e.g.measured as average precision on training data),
4.the number of streams that retrieve a particular document and
5.the ranks of this document within each stream.
Generally,a stronger (i.e.,better performing) stream will more eect on shaping the ®nal
Term weighting across streams using SMART
Stream Weighting scheme
H+M Pairs ltn.nsn
Precision improvements over stems-only retrieval based on TREC-5 data
Streams merged Short queries (%change) Long queries (%change)
All streams +5.4 +20.94
Stems+Phrases+Pairs +6.6 +22.85
Stems+Phrases +7.0 +24.94
Stems+Pairs +2.2 +15.27
Stems+Names +0.6 +2.59
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178168
ranking.A document which is retrieved at a high rank from such a stream is more likely to
end up ranked high in the ®nal result.In addition,the performance of each stream within a
speci®c range of ranks is taken into account.For example,if phrases stream tends to pack
relevant documents between the top 10th and 20th retrieved documents (but not so much into
1±10) we would give premium weights to the documents found in this region of phrase-based
ranking,etc.Table 4 gives some additional data on the eectiveness of stream merging.
Further details are available in our TREC-5 conference article (Strzalkowski et al.,1996).Note
that long text queries bene®t more from linguistic processing.
4.1.Inter-stream merging using precision distribution estimates
We used the following two principal sources of information about each stream to weigh their
relative contributions to the ®nal ranking:
.an actual ranking obtained from a training run (training data,old queries);
.an estimated retrieval precision at certain ranges of ranks.
Precision estimates are used to order results obtained from the streams and this ordering
may vary at dierent rank ranges.Table 5 shows precision estimates for selected streams at
certain rank ranges as obtained from a training collection derived from TREC-4 data.
The ®nal score of a document (d) is calculated using the following formula:
Ai score i d prec franks i jrank i,d 2 ranks i g
where N is the number of streams;A(i ) is the stream coecient (de®ned in the next section);
and score(i )(d) is the normalized score of the document against the query within the stream i;
prec (ranks (i )) is the precision estimate from the precision distribution table for stream i;and
rank (i,d) is the rank of document d in stream i.
We include the precision distribution in streams as well as the ranks.The rank determines
that one document in a stream is better than another in the same stream,but the`speed'with
which we go down diers from stream to stream (some streams lose precision faster than
others and some may gain in some recall regions).
Precision distribution estimates for selected streams
Ranks Stems Phrases Pairs Names
1±5 0.49 0.45 0.33 0.23
6±10 0.42 0.38 0.27 0.18
11±20 0.37 0.32 0.23 0.13
21±30 0.33 0.28 0.21 0.10
31±50 0.27 0.25 0.17 0.08
51±100 0.19 0.17 0.12 0.06
101±200 0.12 0.11 0.08 0.04
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178 169
The stream coecients described in the next section are necessary since the scores are not
necessarily comparable because they may be obtained from dierent engines.The stream
coecients capture a historical eectiveness of a stream which is independent from a particular
precision.However,actual precision is also used to downgrade or upgrade a stream that did
worse or better than the historical average.
For merging purposes,streams are assigned numerical coecients,referred to as A(i) above,
that have two roles:
1.Control the relative contribution of a document score assigned to it within a stream when
calculating the ®nal score for this document.This applies primarily to streams producing
normalized document scores,such as SMART.
2.Change stream-to-stream document score relationships for unnormalized ranking systems,
such as PRISE.
An example of a coecient structure is shown below.They are obtained empirically to
maximize the performance of any speci®c combination of streams.Table 6 summarizes
stream coecient structures used in TREC-5 experiments.Typically,a new combination was
created for a given collection,a retrieval mode (ad hoc vs.routing) and the search engines
5.Query expansion experiments
5.1.Why query expansion?
The purpose of query expansion is to make the user query resemble more closely the
documents it is expected to retrieve.This includes both content,as well as some other aspects
such as composition,style,language type,etc.If the query is indeed made to resemble a
`typical'relevant document,then suddenly everything about this query becomes a valid search
criterion:words,collocations,phrases,various relationships,etc.Unfortunately,an average
search query does not look anything like this,most of the time.It is more likely to be a
Stream merging coecient structures used in TREC-5
stems phrases pairs names
ad hoc gerua1 4 3 3 1
ad hoc gerua3 5 3 3 1
Routing gerou1 4 3 3 1
Routing gesri2 4 3 3 1
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statement specifying the semantic criteria of relevance.This means that except for the semantic
or conceptual resemblance (which we cannot model very well as yet) much of the appearance
of the query (which we can model reasonably well) may be and often is,quite misleading for
search purposes.Where can we get the right queries?
In today's information retrieval,query expansion usually pertains to content and typically is
limited to adding,deleting or reweighting of terms.For example,content terms from
documents judged relevant are added to the query while weights of all terms are adjusted in
order to re¯ect the relevance information.Thus,terms occurring predominantly in relevant
documents will have their weights increased,while those occurring mostly in nonrelevant
documents will have their weights decreased.This process can be performed automatically
using a relevance feedback method,(e.g.,Rocchio,1971),with the relevance information either
supplied manually by the user (Harman 1988),or otherwise guessed,e.g.by assuming top 10
documents relevant,etc.(Buckley,1995).A serious problem with this content-term expansion
is its limited ability to capture and represent many important aspects of what makes some
documents relevant to the query,including particular term co-occurrence patterns and other
hard-to-measure text features,such as discourse structure or stylistics.Additionally,relevance-
feedback expansion depends on the inherently partial relevance information,which is normally
unavailable,or unreliable.Other types of query expansions,including general purpose thesauri
or lexical databases (e.g.,WordNet) have been found generally unsuccessful in information
retrieval (cf.Voorhees,1993,1994).An alternative to term-only expansion is a full-text
expansion which we tried for the ®rst time in TREC-5 (Strzalkowski et al.,1996).In our
approach,queries are expanded by pasting in entire sentences,paragraphs and other sequences
directly from any text document.To make this process ecient,we ®rst perform a search with
the original,unexpanded queries (short queries) and then use top N (10,20) returned
documents for query expansion.These documents are not judged for relevancy,nor assumed
relevant;instead,they are scanned for passages that contain concepts referred to in the query.
Expansion material can be found in both relevant and nonrelevant documents,bene®ting the
®nal query all the same.In fact,the presence of such text in otherwise nonrelevant documents
underscores the inherent limitations of distribution-based term reweighting used in relevance
feedback.Subject to some further`®tness criteria',these expansion passages are then imported
verbatim into the query.The resulting expanded queries undergo the usual text processing
steps,before the search is run again.
Full-text expansion can be accomplished manually,as we did initially to test feasibility of
this approach in TREC-5,or semi-automatically,as we tried this year with excellent results.
Our goal is to fully automate this process.(We did try an automatic expansion in TREC-5,but
it was very simplistic and not very successful,cf.our TREC-5 report (Strzalkowski et al.,1996)
The initial evaluations indicate that queries expanded manually following the prescribed
guidelines are improving the system's performance (precision and recall) by as much as 40% or
more.This appear to be true not only for our own system,but also for other systems:we
asked other groups participating in TREC-5 to run search using our expanded queries and
they reported nearly identical improvements.Below,we describe the three dierent query
expansion techniques explored in TREC-6.
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178 171
5.2.Summarization-based query expansion
We used an automatic text summarizer to derive query-speci®c summaries of documents
returned from the ®rst round of retrieval.The summaries were usually 1 or 2 consecutive
paragraphs selected from the original document text.The purpose was to demonstrate,
in a quick-read abstract,why a given document has been retrieved.If the summary
appeared relevant and moreover captured some new aspect of relevant information,
then it was pasted into the query.Note that it was not important if the document itself was
The summaries were produced automatically using GE Summarizer Tool,a prototype
developed for Tipster Phase 3 project.It works by extracting passages from the document text,
and producing perfectly readable,very brief summaries,at about 5 to 10% of original text
A preliminary examination of TREC-6 results indicate that this mode of expansion is at
least as eective as the purely manual expansion used in TREC-5.This is a very good news,
since we now appear to be a step closer to an automatic expansion.The human-decision factor
has been reduced to an accept/reject decision for expanding the search query with a summary
Ð no need to read the whole document in order to select expansion passages.
5.3.Extraction-based query expansion
We used automatic information extraction techniques to score text passages for presence of
concepts (rather than keywords) identi®ed by the query.Small extraction grammars were
manually constructed for 23 out of 47 routing queries.Using SRI's FASTUS information
extraction system,we selected highest score sentences from known relevant documents in the
training corpus.Please note that this was a routing run,and the setup was somewhat dierent
than in other query expansion runs.In particular,there was only one run against the test
collection (the routing mode allows no feedback).
This run was constructed in collaboration with SRI's team.SRI has developed FASTUS
grammars,run FASTUS over the training documents,scored each sentence,and sent the
sentences to GE.The GE team applied stream model processing to the queries,run the queries
against the test collection and submitted the results to NIST.
5.4.Interactive query expansion with inquery
The results produced at Rutgers were obtained using an interactive system.We believe that
through interaction with the system and the database the user can create signi®cantly better
queries.The support to the user provided by the interface in order to build better queries is at
least as important as any other part of the system.
In our previous contributions we have devoted very signi®cant resources in terms of
processing power and time to the creation of better document representations.In particular,we
have applied NLP techniques to thousands of megabytes of text in order to add less
ambiguous terms to the document representation.In the interaction experiment we attempted
to move processing power and`intelligence'from the representation to the interface.What we
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178172
are trying to do is to spend a few tenths of a second executing even more sophisticated
techniques (including,in future interfaces,NLP) on the query instead of days processing
several gigabytes of the corpus in order to generate a better representation.
A new user interface for InQuery,called RUINQ2,was developed at Rutgers for this
experiment.This is a variation of RUINQ,the InQuery interface developed for use in the
interactive track experiments reported by the Rutgers team (Belkin,1997).
RUINQ2 supports the use of negative and positive feedback.The user is shown a list of 10
document titles at a time.The user can scroll to see another 10 as many times as needed.Any
number of the titles presented can be declared either relevant or non relevant by the user (by
clicking next to the title).When a document is declared relevant (non relevant) some terms are
oered to the user on a positive (negative) feedback window.The user can add to the query
any number of terms from those windows by clicking on the desired term.
RUINQ2 also supported the use of phrases (any sequence of words entered by the user
inside double quotes) and required terms (preceded by a plus sign).
The interactive run was created in order to have a baseline to compare query expansion
using automatically generated summaries,with query expansion using interaction with
document text plus negative/positive feedback.Further experiments based on more re®ned user
interfaces for both systems should help us answer questions such as:which system is easier to
use,which one allows users to create queries faster and which system helps user create more
The interactive run was created by allowing a single user (one of the authors) who had never
seen the topics before,to interact with the system for no more than 15 min per topic in order
to build the corresponding query.When the user was satis®ed with the query he would click on
a button that would print out the rankings of (at most) 1000 documents in TREC format.In
several cases less than 1000 documents were found.
6.Summary of results
6.1.Ad hoc runs
Ad hoc retrieval is when an arbitrary query is issued to search a database for relevant
documents.In a typical ad hoc search situation,a query is used once,then discarded,thus
leaving little room for optimization.Our ad hoc experiments were conducted in several
subcategories,including automatic,manual and using dierent sizes of databases and dierent
types of queries.An automatic run means that there was no human intervention in the process
at any time.A manual run means that some human processing was done to the queries and
possibly multiple test runs were made to improve the queries.A short query is derived using
only one section of a TREC-5 topic,namely the Description ®eld.A full query is derived from
any or all ®elds in the topic.An example TREC-5 query is show below;note that the
Description ®eld is what one may reasonably expect to be an initial search query,while
Narrative provides some further explanation of what relevant material may look like.The Title
®eld provides a single concept of interest to the searcher;it was not permitted in the short
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178 173
Define Argentine and British international relations
It has been 15 years since the war between Argentina and the United
Kingdom in 1982 over sovereignty in the Falkland Islands.A relevant
report will describe their relations after that period.Any kind of
international contact between the two countries is relevant,to
include commercial,economic,cultural,diplomatic,or military
exchanges.Negative reports on the absence of such exchanges are
also desirable.Reports containing information on direct exchanges
between Argentina and the Falkland Islands are also relevant.
Table 7 summarizes selected runs performed with our NLIR system on the TREC-6
database using 50 queries numbered 301 through 350.The SMART baselines were produced
by Cornell-SaBir team using version 11 of the system.The rightmost column is an unocial
rerun of the GERUA1 after ®xing of a simple bug.Using our version of InQuery-based system
at Rutgers on the same set of queries and the same database we observed consistently large
improvements in retrieval precision attributed to the expanded queries.The results labeled
``Best NL''correspond to the best results we obtained using natural language processing
Precision improvement in NLIR system vs.SMART (v.11) baselines
Full man long man longÿ1
prec.smart best NL smart best NL best NL
11pt.Avg 0.1429 0.1837 0.2672 0.2783 0.2859
%change +28.5 +87.0 +94.7 +100.0
@10 docs 0.3000 0.3840 0.5060 0.5200 0.5200
%change +28.0 +68.6 +73.3 +73.3
@30 docs 0.2387 0.2747 0.3887 0.3933 0.3940
%change +15.0 +62.8 +64.7 +65.0
@100 doc 0.1600 0.1736 0.2480 0.2598 0.2574
%change +8.5 +55.0 +62.3 +60.8
Recall 0.57 0.53 0.61 0.58 0.62
%change ÿ7.0 +7.0 +1.7 +8.7
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Routing is a process in which a stream of previously unseen documents are ®ltered and
distributed among a number of standing pro®les,also known as routing queries.In routing,
documents can be assigned to multiple pro®les.In categorization,a type of routing,a single
best matching pro®le is selected for each document.Routing is harder to evaluate in a
standardized setup than the retroactive retrieval because of its dynamic nature,therefore a
simulated routing mode has been used in TREC.A simulated routing mode (TREC-style)
means that all routing documents are available at once,but the routing queries (i.e.,terms and
their weights) are derived with respect to a dierent training database,speci®cally TREC
collections from previous evaluations.This way,no statistical or other collection-speci®c
information about the routing documents is used in building the pro®les and the participating
systems are forced to make assumptions about the routing documents just like they would in
real routing.However,no real routing occurs,and the prepared routing queries are run against
the routing database much the same way they would be in an ad hoc retrieval.Documents
retrieved by each routing query,ranked in order of relevance,become the content of its
6.2.1.Query development against the training collection
In Smart routing,automatic relevance feedback was performed to build routing queries
using the training data available from previous TRECs.The routing queries,split into streams,
were then run against stream-indexed routing collection.The weighting scheme was selected in
such a way that no collection-speci®c information about the current routing data has been
used.Instead,collection-wide statistics,such as idf weights,were those derived from the
training data.The routing was carried out in the following four steps:
1.A subset of the previous TREC collections was chosen as the training set and four index
streams were built.Queries were also processed and run against the indexes.For each query,
1000 documents are retrieved.The weighting schemes used were:lnc.ltc for stems,ltc.ntc for
phrases,ltc.ntc for head+modi®er pairs and ltc.ntc for names.
2.The ®nal query vector was then updated through an automatic feedback step using the
known relevance judgements.Up to 350 terms occurring in the most relevant documents
were added to each query.Two alternative expanded vectors were generated for each query
using dierent sets of Rocchio parameters.
3.For each query,the best performing expansion was retained.These were submitted to NIST
as ocial routing queries.
Precision averages for 47 routing queries (TREC-6 routing data)
Runs 11pt.Prec At 5 docs At 10 docs R-Prec
main routing,gerou1 0.2702 0.5532 0.4787 0.3176
query expansion gesri2 0.2458 0.5447 0.4894 0.2906
reranked gerou1,srige1 0.2730 0.5574 0.5021 0.3126
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178 175
4.The ®nal queries were run against the four-stream routing test collection and retrieved
results were merged.
6.2.2.Reranking using rescoring via extraction
The gesri run shown on Table 8 was created at SRI using the output from GE's main
routing run.SRI's FASTUS (Hobbs,1996) was used to score documents retrieved and rerank
them if they contained concepts asked for in the query.For details of this run please refer to
SRI's chapter (Bear,Israel,Petit & Martin,1997)
The results of using information extraction techniques are shown in Table 8 and compared
to our main routing run.The SRI runs use a dierent type of expansion (using sentences
selected through a concept extraction system (FASTUS)) than the expansion we have described
in the rest of this paper.This kind of expansion did not seem to work as well as we expected.
On the other hand,we note a slight improvement in average precision and a more de®nite
precision improvement near the top of the ranking in FASTUS rescoring run.This is only a
®rst attempt at a serious-scale experiment of this kind and the results are de®nitely
We presented in some detail our natural language information retrieval system consisting of
an advanced NLP module and a`pure'statistical core engine.While many problems remain to
be resolved,including the question of adequacy of term-based representation of document
content,we attempted to demonstrate that the architecture described here is nonetheless viable.
In particular,we demonstrated that natural language processing can now be done on a fairly
large scale and that its speed and robustness has improved to the point where it can be applied
to real IR problems.
The main observation to make is that thus far natural language processing has not proven as
eective as we would have hoped in to obtain better indexing and better term representations
of queries.Using linguistic terms,such as phrases,head-modi®er pairs,names,does help to
improve retrieval precision,but the gains remain quite modest.On the other hand,full text
query expansion works remarkably well and even more so in combination with linguistic
indexing.Our main eort in the immediate future will be to explore ways to achieve at least
partial automation of this process.Using information extraction techniques to improve
retrieval either by building better queries,or by reorganizing the results is another promising
line of investigation.
We would like to thank Donna Harman for making the NIST's PRISE system available to
this project since the beginning of TREC.We also thank Chris Buckley for helping us to
J.Perez-Carballo,T.Strzalkowski/Information Processing and Management 36 (2000) 155±178176
understand the inner workings of SMART.We would like to thank Ralph Weischedel for
providing and assisting in the use of the BBN's part of speech tagger.Finally,thanks to SRI's
Jerry Hobbs,David Israel and John Bear for their collaboration on joint experiments.This
paper is based upon work supported in part by the Defense Advanced Research Projects
Agency under Tipster Phase-3 Contract 97-F157200-000.
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