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Clustering in Swedish: The Impact of some Properties of the Swedish Language on Document Clustering and an Evaluation Method
KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
2005 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Text clustering divides a set of texts into groups, so that texts within each group are similar in content. It may be used to uncover the structure and content of unknown text sets as well as to give new perspectives on known ones. The contributions of this thesis are an investigation of text representation for Swedish and an evaluation method that uses two or more manual categorizations.

Text clustering, at least such as it is treated here, is performed using the vector space model, which is commonly used in information retrieval. This model represents texts by the words that appear in them and considers texts similar in content if they share many words. Languages differ in what is considered a word. We have investigated the impact of some of the characteristics of Swedish on text clustering. Since Swedish has more morphological variation than for instance English we have used a stemmer to strip suffixes. This gives moderate improvements and reduces the number of words in the representation.

Swedish has a rich production of solid compounds. Most of the constituents of these are used on their own as words and in several different compounds. In fact, Swedish solid compounds often correspond to phrases or open compounds in other languages.In the ordinary vector space model the constituents of compounds are not accounted for when calculating the similarity between texts. To use them we have employed a spell checking program to split compounds. The results clearly show that this is beneficial.

The vector space model does not regard word order. We have tried to extend it with nominal phrases in different ways. Noneof our experiments have shown any improvement over using the ordinary model.

Evaluation of text clustering results is very hard. What is a good partition of a text set is inherently subjective. Automatic evaluation methods are either intrinsic or extrinsic. Internal quality measures use the representation in some manner. Therefore they are not suitable for comparisons of different representations.

External quality measures compare a clustering with a (manual) categorization of the same text set. The theoretical best possible value for a measure is known, but it is not obvious what a good value is -- text sets differ in difficulty to cluster and categorizations are more or less adapted to a particular text set. We describe an evaluation method for cases where a text set has more than one categorization. In such cases the result of a clustering can be compared with the result for one of the categorizations, which we assume is a good partition. We also describe the kappa coefficient as a clustering quality measure in the same setting.

Abstract [sv]

Textklustring delar upp en mängd texter i grupper, så att texterna inom dessa liknar varandra till innehåll. Man kan använda textklustring för att uppdaga strukturer och innehåll i okända textmängder och för att få nya perspektiv på redan kända. Bidragen i denna avhandling är en undersökning av textrepresentationer för svenska texter och en utvärderingsmetod som använder sig av två eller fler manuella kategoriseringar.

Textklustring, åtminstonde som det beskrivs här, utnyttjar sig av den vektorrumsmodell, som används allmänt inom området. I denna modell representeras texter med orden som förekommer i dem och texter som har många gemensamma ord betraktas som lika till innehåll. Vad som betraktas som ett ord skiljer sig mellan språk. Vi har undersökt inverkan av några av svenskans egenskaper på textklustring. Eftersom svenska har större morfologisk variation än till exempel engelska har vi tagit bort suffix med hjälp av en stemmer. Detta ger lite bättre resultat och minskar antalet ord i representationen.

I svenska används och skapas hela tiden fasta sammansättningar. De flesta delar av sammansättningar används som ord på egen hand och i många olika sammansättningar. Fasta sammansättningar i svenska språket motsvarar ofta fraser och öppna sammansättningar i andra språk. Delarna i sammansättningar används inte vid likhetsberäkningen i vektorrumsmodellen. För att utnyttja dem har vi använt ett rättstavningsprogram för att dela upp sammansättningar. Resultaten visar tydligt att detta är fördelaktigt

I vektorrumsmodellen tas ingen hänsyn till ordens inbördes ordning. Vi har försökt utvidga modellen med nominalfraser på olika sätt. Inga av våra experiment visar på någon förbättring jämfört med den vanliga enkla modellen.

Det är mycket svårt att utvärdera textklustringsresultat. Det ligger i sakens natur att vad som är en bra uppdelning av en mängd texter är subjektivt. Automatiska utvärderingsmetoder är antingen interna eller externa. Interna kvalitetsmått utnyttjar representationen på något sätt. Därför är de inte lämpliga att använda vid jämförelser av olika representationer.

Externa kvalitetsmått jämför en klustring med en (manuell) kategorisering av samma mängd texter. Det teoretiska bästa värdet för måtten är kända, men vad som är ett bra värde är inte uppenbart -- mängder av texter skiljer sig åt i svårighet att klustra och kategoriseringar är mer eller mindre lämpliga för en speciell mängd texter. Vi beskriver en utvärderingsmetod som kan användas då en mängd texter har mer än en kategorisering. I sådana fall kan resultatet för en klustring jämföras med resultatet för en av kategoriseringarna, som vi antar är en bra uppdelning. Vi beskriver också kappakoefficienten som ett kvalitetsmått för klustring under samma förutsättningar.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , vii, 35 p.
Series
Trita-NA, ISSN 0348-2952 ; 05:31
Keyword [en]
Document Clustering
National Category
Language Technology (Computational Linguistics)
Identifiers
URN: urn:nbn:se:kth:diva-438ISBN: 91-7178-166-8 (print)OAI: oai:DiVA.org:kth-438DiVA: diva2:12039
Presentation
2005-10-18, E3, E-huset, Osquars backe 14, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20101220Available from: 2005-09-29 Created: 2005-09-29 Last updated: 2018-01-13Bibliographically approved
List of papers
1. Improving Clustering of Swedish Newspaper Articles using Stemming and Compound Splitting
Open this publication in new window or tab >>Improving Clustering of Swedish Newspaper Articles using Stemming and Compound Splitting
2003 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The use of properties of the Swedish language when indexing newspaper articles improves clustering results. To show this a clustering algorithm was implemented and language specific tools were used when building the representation of the articles.Since Swedish is an inflecting language many words have different forms. Thus two documents compared based on word occurrence(i.e. the vector space model and cosine measure of Information Retrieval) do not necessarily become similar although containing the sameword(s). To overcome this we have used a stemmer.Compounds are regularly formed as one word in Swedish. Hence indexing on words leaves the informationin the components of compounds unused.We use the spell checking program Stavato split compounds into their components.Newspapers sort their articles into sections such as Economy, Domestic, Sports etc. Using these we calculate entropy for the clusterings and use as a measure of quality.We have found that stemming improves clustering results on our collections by about 4 % compared to not using it. Compound splitting improves results by about 10 % (by 13 % incombination with stemming). Keeping the original compounds in the representation does not improve results.

 

 

 

National Category
Computer Sciences
Identifiers
urn:nbn:se:kth:diva-7120 (URN)
Conference
NoDaLiDa 2003, Reykjavik, Iceland 2003
Note
QC 20100806Available from: 2005-09-29 Created: 2005-09-29 Last updated: 2018-01-13Bibliographically approved
2. Comparing Comparisons: Document Clustering Evaluation Using Two Manual Classifications
Open this publication in new window or tab >>Comparing Comparisons: Document Clustering Evaluation Using Two Manual Classifications
2004 (English)Conference paper, Published paper (Refereed)
Abstract [en]

“Describe your occupation in a few words”, is a question answered by 44 000 Swedish twins.Each respondent was then manually categorized according to two established occupation classificationsystems. Would a clustering algorithm have produced satisfactory results? Usually,this question cannot be answered. The existing quality measures will tell us how much thealgorithmic clustering deviates from the manual classification, not if this is an acceptable deviation. But in our situation, with two different manual classifications (in classificationsystems called AMSYK and YK80), we can indeed construct such quality measures. If the algorithmic result differs no more from the manual classifications than these differ from eachother (comparing the comparisons) we have an indication of its being useful. Further, weuse the kappa coefficient as a clustering quality measure. Using one manual classification asa coding scheme we assess the agreement of a clustering and the other. After applying both these novel evaluation methods we conclude that our clusterings are useful.

National Category
Computer Sciences
Identifiers
urn:nbn:se:kth:diva-7121 (URN)
Conference
ICON 2004, India.
Note
QC 20100806Available from: 2005-09-29 Created: 2005-09-29 Last updated: 2018-01-13Bibliographically approved
3. The Impact of Phrases in Document Clustering for Swedish
Open this publication in new window or tab >>The Impact of Phrases in Document Clustering for Swedish
2005 (English)In: Proceedings of the 15th NODALIDA conference, Joensuu 2005 / [ed] Werner, S., 2005, 173-179 p.Conference paper, Published paper (Refereed)
Abstract [en]

We have investigated the impact of using phrases in the vector spacemodel for clustering documents in Swedish in different ways. The investigation is carried out on two textsets from different domains: one set of newspaper articles and one set of medical papers.The use of phrases do not improveresults relative the ordinary use ofwords. The results differ significantly between the text types. Thisindicates that one could benefit from different text representations for different domains although a fundamentally different approach probably would be needed.

National Category
Computer Sciences
Identifiers
urn:nbn:se:kth:diva-7122 (URN)952-458-771-8 (ISBN)
Conference
NoDaLiDa 2005, Joensuu, Finland, 2005
Note
QC 20100806Available from: 2005-09-29 Created: 2005-09-29 Last updated: 2018-01-13Bibliographically approved

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