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Automatic Generation of English/Chinese ThesaurusBased on a Parallel Corpus in Laws

Christopher C. YangDepartment of Systems Engineering and Engineering Management, The Chinese University of Hong Kong,Shatin, New Territories, Hong Kong, E-mail:

Johnny LukDepartment of Computer Science and Information Systems, The University of Hong Kong, Hong Kong.

The information available in languages other than Englishin the World Wide Web is increasing significantly. Accord-ing to a report from Computer Economics in 1999, 54% ofInternet users are English speakers (English Will Domi-nate Web for Only Three More Years, Computer Eco-nomics, July 9, 1999, However, it is predicted thatthere will be only 60% increase in Internet users amongEnglish speakers verses a 150% growth among non-English speakers for the next five years. By 2005, 57% ofInternet users will be non-English speakers. A report in 2000 showed that the number of Internetusers in China had been increased from 8.9 million to16.9 million from January to June in 2000 (Report:China Internet users double to 17 million,,July, 2000, According to Nielsen/NetRatings, there was a dramatic leap from 22.5 millionsto 56.6 millions Internet users from 2001 to 2002. Chinahad become the second largest global at-home Internetpopulation in 2002 (USs Internet population was 166millions) (Robyn Greenspan, China Pulls Ahead of Japan,, April 22, 2002,,,5911_1013841,00.html). All of the evidences reveal the importance of cross-lingual research to satisfy the needs in the near future.

Digital library research has been focusing in struc-tural and semantic interoperability in the past. Searchingand retrieving objects across variations in protocols,formats and disciplines are widely explored (Schatz, B.,& Chen, H. (1999). Digital libraries: technological ad-vances and social impacts. IEEE Computer, Special Is-sue on Digital Libraries, February, 32(2), 4550.; Chen, H.,Yen, J., & Yang, C.C. (1999). International activities: de-velopment of Asian digital libraries. IEEE Computer,Special Issue on Digital Libraries, 32(2), 4849.). How-ever, research in crossing language boundaries, espe-cially across European languages and Oriental lan-guages, is still in the initial stage. In this proposal, we putour focus on cross-lingual semantic interoperability by

developing automatic generation of a cross-lingual the-saurus based on English/Chinese parallel corpus. Whenthe searchers encounter retrieval problems, profes-sional librarians usually consult the thesaurus to identifyother relevant vocabularies. In the problem of searchingacross language boundaries, a cross-lingual thesaurus,which is generated by co-occurrence analysis and Hop-field network, can be used to generate additional se-mantically relevant terms that cannot be obtained fromdictionary. In particular, the automatically generatedcross-lingual thesaurus is able to capture the unknownwords that do not exist in a dictionary, such as names ofpersons, organizations, and events. Due to Hong Kongsunique history background, both English and Chineseare used as official languages in all legal documents.Therefore, English/Chinese cross-lingual information re-trieval is critical for applications in courts and the gov-ernment. In this paper, we develop an automatic thesau-rus by the Hopfield network based on a parallel corpuscollected from the Web site of the Department of Justiceof the Hong Kong Special Administrative Region(HKSAR) Government. Experiments are conducted tomeasure the precision and recall of the automatic gen-erated English/Chinese thesaurus. The result shows thatsuch thesaurus is a promising tool to retrieve relevantterms, especially in the language that is not the same asthe input term. The direct translation of the input termcan also be retrieved in most of the cases.

1. Introduction

As the Web-based information systems in languagesother than English are growing exponentially, the demandfor searching across language boundaries is obvious. We areexpecting that the number of Internet users who are non-English speakers will increase to more than half of thepopulation. The Digital Library Initiative funded by NSF/DARPA/NASA in the U.S. has laid the foundation forsearching and retrieving objects across variations in proto-cols and formats, structural interoperability, and acrossdifferent disciplines, semantic interoperability (Chen,

2003 Wiley Periodicals, Inc.


1998). However, the work done on linguistic interoperabil-ity is relatively less. A few cross-lingual information re-trieval works have been done, mostly focused on Europeanlanguages, such as English, Spanish, German, French, andItalian. Relatively less cross-lingual information retrievalworks have been done on Oriental language or across Eu-ropean and Oriental languages. The difficulties of cross-lingual information retrieval between European and Orien-tal languages are comparatively higher than the difficultiesamong European languages. These are due to several rea-sons: 1) machine translation techniques between Europeanand Oriental languages are rather less mature, 2) parallel/comparable corpus in European and Oriental languages arecomparatively rare, 3) the grammar of European and Ori-ental languages are significantly different. As English andChinese are the most popular languages in the world, thedesire on an efficient and effective Chinese-English cross-lingual information retrieval system is significant althoughthere are still many research problems to be solved.

1.1 Cross-Lingual Information Retrieval

Cross-lingual information retrieval (CLIR) refers to theability to process a query for information in one language,search a collection of objects, including text, images, audiofiles, etc. and return the most relevant objects, translatedinto the users language if necessary (Klavans et al., 1999;Oard & Dorr, 1996). For users who are able to read morethan one language, CLIR systems allow them to submit onequery and search for documents in more than one languageinstead of submitting multiple queries in different lan-guages. For users who are able to read more than onelanguage but may not be able to write fluently in othernon-native languages, the CLIR systems help them tosearch across the language boundaries without them ex-pressing their queries in the language that they are notfamiliar with. Even for users who only read a single lan-guage, the CLIR systems help them to retrieve a smallportion of relevant documents from the collection of docu-ments in other languages, and therefore fewer documentsare examined by users and being translated. Current re-search in cross-lingual information retrieval can be dividedinto two major approaches: (1) controlled vocabulary and(2) free text.

In the controlled vocabulary approach, documents aremanually indexed using a predetermined vocabulary andqueries from users are using terms drawn from the samevocabulary (Oard & Dorr, 1996). Systems exploiting suchapproach bring queries and documents into a representationspace by use of a multilingual thesaurus to relate the se-lected terms from each language to a common set of lan-guage-independent concept identifier, where the documentselection is based on the concept identifier matching (Davis& Dunning, 1995; Fluhr, 1995; Flur & Radwan, 1993;Radwan & Fluhr, 1995). However, it imposes the limitationof the user-employed vocabulary and the selection of the-saurus highly affects the performance of the retrieval. Al-

though such approach is widely used in commercial andgovernment applications, it is not practical as the number ofconcepts increases and becomes less manageable. The per-formance becomes worse as the collection of documentsincreases, such as the World Wide Web, because the doc-uments generated from diverse sources cannot be standard-ized easily. Besides, the training of users to select searchterms from the controlled vocabulary is difficult. Generalusers are usually unable to identify the appropriate term torepresent their information needs.

The free text approach does not limit the usage of vo-cabulary but using the words that appear in the documents.Queries or documents are translated and then retrieval isperformed. The free text approach can be further dividedinto two approaches: (1) knowledge-based approach and (2)corpus-based approach. Knowledge-based approach em-ploys ontology or dictionary (Ballesteros & Crosft, 1997;Hull & Grefenstette, 1996; Oard, 1997; Radwan & Fluhr,1995). Ontology is an inventory of concepts, which is or-ganized under some internal structuring principle. Multilin-gual thesaurus is an example of ontology to organize ter-minology from more than one language. Complex thesauri,which encode syntactic and semantic information aboutterms, are used as concept index in automatic text retrievalsystems (Oard & Dorr, 1996). Dictionary replaces eachterm in the query with an appropriate term or set of terms inthe target language Salton, 1970). However, the knowledge-based approach encounters several problems. Terms may beambiguous and may have multiple meanings. The thesaurusor dictionary may not has terms that are essential for acorrect interpretation of the query. For examples, technicalterms, abbreviation, names of persons, organization orevents may not be included in the thesaurus or dictionary.There are also new terms entering the lexicon all the timesince language usage is a creative activity.

Corpus-based approach overcomes the limitation of theknowledge-based approach by making use of the statisticalinformation of term usage in parallel or comparable corporato construct an automatic thesaurus. Since it is impracticalto construct a bilingual dictionary or sophisticated multilin-gual thesauri manually for large applications, the corpus-based approach uses the term co-occurrence statistics acrosslarge document collections to construct a statistical transla-tion model for cross-lingual information retrieval. Severalworks have been done based on this approach. Landauer andLittman use Latent Semantic Indexing (LSI) to construct amulti-lingual semantic space [Dumais et al, 1997; Landauer& Littman, 1990). Experiments are conducted on a French-English collection. Davis and Dunning (1995a, b) applyevolutionary programming on a Spanish-English collection.Sheridan and Ballerini (1996) apply thesaurus-based queryexpansion techniques on a comparable Italian-English col-lection. However, there is a lack of corpus-based approachbeing applied between European and Oriental languages, inparticular between English and Chinese, mainly due to theirsignificant difference in grammar and low availability ofsuch corpus.


2. Multilingual Corpus

Multilingual corpus is a collection of text in electronicform (written language corpus) where texts in differentlanguages are put together either based on parallelism orcomparability. Multilingual corpus constructed based onparallelism and comparability are known as parallel corpusand comparable corpus, respectively. Parallel corpus can bedeveloped using overt translation or covert translation. Theovert translation posses a directional relationship betweenthe pair of texts in two languages, which means texts inlanguage A (source text) is translated into texts in languageB (translated text) (Rose, 1981). The covert translation isnon-directional. Multilingual documents expressing thesame content in different languages are generated by thesame source (Leonardi, 2000). Therefore, none of the text ineach pair of such parallel corpus is marked as translated textor source text.

Comparable corpus consists of texts in multiple lan-guages composed independently, which shared similar cri-teria of content, domain, and communicative function. Cri-teria for creating comparable corpora depends on the ho-mogeneity of texts, both with or across languages, in termsof features such as subject domain, author-reader relation-ship, text origin and constitution, factuality, technicality andintended outcome (Zanettin, 1998).

Several techniques have been developed to constructparallel corpus automatically. The most prominent systemthat generate parallel corpus from the World Wide Web isStructural Translation Recognition for Acquiring NaturalData (STRAND), developed by Resnik (1998, 1999).STRAND consists of three modules, candidate generationmodule, candidate evaluation module and candidate pairfiltering module. A similar system has also been developedby Nie (Nie et al., 1999). However, these systems focus ongenerating parallel corpus in European languages only.

2.1 Automatic Cross-lingual Thesaurus

In this paper, we develop a English/Chinese cross-lingualthesaurus based on a English/Chinese parallel corpus col-lected from the Bilingual Legal Information System (BLIS)of the HKSAR Government. Part of the BLIS parallelcorpus is based on the overt translation and part of it isbased on covert translation depending on the date of thedocument released. Using the co-occurrence analysis ofEnglish and Chinese terms extracted in the parallel corpus,the synaptic weights of the Hopfield network are deter-mined. The cross-lingual thesaurus is generated automati-cally by training in the Hopfield network. Such cross-lin-gual thesaurus supports cross-lingual information retrievalby suggesting semantically relevant terms in different lan-guages so that human users who are not familiar with thelanguages are able to obtain better retrieval performance.

3. Automatic Generation of English/ChineseThesaurus

Unlike the controlled vocabulary approach and knowl-edge-based approach, the cross-lingual thesaurus is gener-ated from the parallel corpus automatically without humanintervention in the corpus-based approach. Knowledge rep-resentations are created from the corpus without pre-definedlanguage representation and linguistic information by hu-man experts. The automatic Chinese-English thesaurus gen-eration system basically consists of five components: i) theparallel/comparable Chinese-English corpus, ii) Englishterm extraction, iii) Chinese term extraction, iv) co-occur-rence analysis, and v) Hopfield network as shown in Figure1. Important terms (or concept descriptors) are first ex-tracted from the parallel or comparable corpus automati-cally using the English term extraction and Chinese term

FIG. 1. Automatic thesaurus generated from a parallel/comparable corpus.


extraction modules. To be more precise, a concept is arecognizable un...


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