A Fast and Simple Stemming Algorithm forGerman Words1
Jörg Caumanns
Free University of Berlin, CeDiS
caule@gmx.de

Abstract

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1.1 Stemming
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1.2 Stemming German Texts
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SDUWRIWKHVWHP DVLQ/HEHU •

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2.1 Recursive Context Free Suffix-Stripping
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VWHPPLQJ 6WHPPLQJ HUURUV DV JLYHQ LQ URZ IRXU DQG ILYH DUH SUHYHQWHG E\ WKH ILUVW SDUW RI WKH
VWHPPLQJ DOJRULWKP  VXEVWLWXWLRQ (UURUV DV LQ OLQH VL[ DQG VHYHQ DUH WKH SULFH IRU WKH DOJRULWKP V
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2.2 Substitution-XVWGRLQJVXIIL[VWULSSLQJZLWKRXWWDNLQJDQ\FRQWH[WXDODQGOLQJXLVWLFLQIRUPDWLRQLQWRDFFRXQWOHDGV
WRWZRPDLQVWHPPLQJHUURUV•,Q*HUPDQODQJXDJHPDQ\SOXUDOIRUPVDUHEXLOWE\FKDQJLQJDYRZHOWRDQ8PODXW l|
DQGRUUHSODFLQJE\VV7KHVHFKDQJHVRIWKHVWHPFDQQRWEHKDQGOHGE\VXIIL[VWULSSLQJ
7KHVDPHKROGVIRULUUHJXODUYHUEIRUPV HJKDOWHQKLHOW •&KDUDFWHUVHTXHQFHVWKDWEHORQJWRJHWKHUEHFDXVHWKH\IRUPDVLQJOHVRXQG HJVFKHL
DQGLH DUHVSOLWDQG SDUWO\ FXWRIIE\WKHVWULSSLQJDOJRULWKP

7RSUHYHQWWKHVHVWHPPLQJHUURUVVRPHVXEVWLWXWLRQVDUHGRQHEHIRUHVXIIL[VWULSSLQJ (DFK8PODXWLVUHSODFHGE\LWVFRUUHVSRQGLQJYRZHODQGLVVXEVWLWXWHGE\VV 7KHVHFRQGFKDUDFWHURIHDFKGRXEOHFKDUDFWHULVUHSODFHGE\  VFKFKHLDQGLHDUHUHSODFHGE\VSHFLDOFKDUDFWHUV   
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GLVFULPLQDWRU$VWKHQXPEHURIWKHVHHUURUVLVYHU\ORZ RQO\RIWKHVHSDLUVZHUHIRXQGLQDOLVWRI
PRUHWKDQQRXQV WKHUHLVQRHIIRUWGRQHWRKDQGOHWKHP7KHVHFRQGDQGWKLUGUHSODFHPHQWV
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7KH DOJRULWKP FDQ EH LPSURYHG E\ VXEVWLWXWLQJ RWKHU FKDUDFWHUV WRR HJ [ IRU ] WR KDQGOH
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7KHRQO\UHTXLUHPHQWIRUDQ\VXEVWLWXWLRQLVWKDWLWVKRXOGHLWKHUUHVXOWLQDQRWKHUPRUSKRORJLFDOIRUP
RIWKHVDPHZRUGRUSURGXFHDQRQZRUG,IWKLVUHTXLUHPHQWLVQRWPHWDVWHPPLQJHUURUZLOORFFXU(JVXEVWLWXWLQJIRUVVLQ%XHOHDGVWR%XVVHZKLFKLVWKHSOXUDOIRUPRI%XV)RUWKLVUHDVRQ%XVDQG%XHDUHERWKVWHPPHGWRWKHVDPHGLVFULPLQDWRU%XV
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3 Evaluation
7KHDOJRULWKPGHVFULEHGLQWKLVUHSRUWKDVEHHQWHVWHGZLWKDOLVWRIDERXWWHUPVWDNHQIURPWKH*HUPDQYHUVLRQRIWKH/LQX[VSHOOFKHFNHU$GGLWLRQDOWHVWVKDYHEHHQSHUIRUPHGZLWKVFLHQWLILFWH[WV
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WZRRUHPRUHGLIIHUHQWWHUPVDUHFRQIODWHGWRWKHVDPHGLVFULPLQDWRU RYHUVWHPPLQJ(UURUVRIHLWKHUNLQGZLWKLQDJLYHQWH[WRUZRUGOLVWVXPXSWRWKHRYHUDOOHUURUUDWHRIWKHVWHPPLQJ
DOJRULWKP

3.1 Understemming Errors(YHQWKRXJKWKHVWHPPLQJDOJRULWKPWHQGVWRVWULSRIIPRUHOHWWHUVWKDQQHFHVVDU\LWPD\KDSSHQWKDW
QRWWKHZKROHGHFOLQDWLRQDOVXIIL[RIDWHUPLVUHPRYHG7KLVNLQGRIHUURURFFXUVZLWKWKUHHJURXSVRIZRUGV•,UUHJXODU YHUEV DQG DOO RI WKHLU FRPSRXQGLQJV  DUH QRW UHGXFHG WR WKH VDPH GLVFULPLQDWRU
7KH'XGHQOLVWVRIWKHVHYHUEVLQFOXGLQJPDQ\RIWKHPRVWFRPPRQO\XVHG•)HPDOHIRUPVRISURIHVVLRQVDUHLQPDQ\FDVHVQRWFRQIODWHGWRWKHVDPHGLVFULPLQDWRU(J6FKDXVSLHOHULQ DFWUHVV  LV QRW VWHPPHG DW DOO ZKLOH 6FKDXVSLHOHULQQHQ LV FRQIODWHG WR
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0DQ\ ZRUGV ZLWK D *UHHN RU /DWLQ RULJLQ DUH QRW FRQIODWHG FRUUHFWO\ ([DPSOHV IRU WKHVH
NLQGVRIZRUGVDUH'UDPD 3OXUDO'UDPHQ DQG0LQLPXP 3OXUDO0LQLPD 

7KH VHFRQG NLQG RI HUURU FDQ HDVLO\ EH SUHYHQWHG E\ VXEVWLWXWLQJ HDFK RFFXUUHQFH RI HULQQ ZLWKHULQ:LWKWKH/LQX[ZRUGOLVWLWZRXOGUHGXFHWKHDPRXQWRIVWHPPLQJHUURUVE\ZLWKRXWFDXVLQJ
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KDYHEHHQFKHFNHGQRZRUGRIWKLVNLQGRFFXUUHG)RUWKLVUHDVRQLWZDVGHFLGHGQRWWRGRWKHHULQQ HULQ UHSODFHPHQW DV WKH DGGLWLRQDO FRPSXWDWLRQDO WLPH LV QRW MXVWLILHG E\ WKDW PLQLPXP HIIHFW(UURUVRIWKHILUVWDQGWKLUGNLQGVFDQRQO\EHSUHYHQWHGE\XVLQJDGLFWLRQDU\$)DVWDQG6LPSOH6WHPPLQJ$OJRULWKPIRU*HUPDQ:RUGV,QJHQHUDOLWFRXOGEHVDLGWKDWFDVHVZKHUHWZRGLIIHUHQWPRUSKRORJLFDOIRUPVRIWKHVDPHZRUGDUH
VWHPPHG WR GLIIHUHQW GLVFULPLQDWRUV DUH YHU\ UDUH IRU QRXQV DQG DGMHFWLYHV *LYLQJ D TXDOLWDWLYH
HVWLPDWHIRUWKHDPRXQWRIXQGHUVWHPPLQJHUURUVRQYHUEVLVGLIILFXOW7KHYDVWPDMRULW\RIDOOYHUEVDUH UHJXODU DQG HDV\ WR VWHP %XW WKH IHZ LUUHJXODU YHUEV WKDW FDXVH XQGHUVWHPPLQJ HUURUV DUH YHU\
FRPPRQLQVSRNHQDQGZULWWHQ*HUPDQ,QDZRUGOLVWMXVWRIDOOYHUEVPD\EHLUUHJXODUEXWLQD
VLQJOHWH[WWKHLUDPRXQWFRXOGEHDQGPRUH'HSHQGLQJRQWKHSXUSRVHWKHVXEVWLWXWHDQGVWHP
DOJRULWKPLVXVHGIRULUUHJXODUYHUEVPD\FDXVHDQHUURUUDWHVRPHZKHUHEHWZHHQDQG DERXW RI DOO *HUPDQ ZRUGV DUH YHUEV  $V WKLV HUURU UDWH PD\ QRW EH DFFHSWDEOH D VROXWLRQ IRU WKH
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3.2 Overstemming Nouns
7KH FUXFLDO DVSHFW IRU WKH TXDOLW\ RI FRQWH[WIUHH VXIIL[ VWULSSLQJ LV WKH DPRXQW RI RYHUVWHPPLQJ
HUURUVWKDWLVWKHQXPEHURIGLIIHUHQWZRUGVWKDWDUHFRQIODWHGWRWKHVDPHGLVFULPLQDWRU
7KHILUVWH[SHULPHQWDOWHVWZDVGRQHRQQRXQVE\RQO\UHPRYLQJWKHVXIIL[HVHQVDQGHU
7KHUHVXOWZDVH[WUHPHO\SRVLWLYH7KH/LQX[VSHOOFKHFNHU VZRUGOLVWFRQWDLQVQRXQVVRPHRI
WKHP LQ ERWK VLQJXODU DQG SOXUDO IRUPV 7KHVH WHUPV ZHUH FRQIODWHG LQWR  GLIIHUHQW
GLVFULPLQDWRUVE\WKHVWHPPLQJDOJRULWKP2IWKHVHGLVFULPLQDWRUVRQO\  ZHUHQRWXQLTXH2IWKHQRXQVRQO\  ZHUHQRWVWHPPHGWRDXQLTXHGLVFULPLQDWRU,QRQH
FDVH  GLIIHUHQW ZRUGV ZHUH VWHPPHG WR WKH VDPH GLVFULPLQDWRU %XFK %FKVH %XFKVH DQG
%FKQHU LQFDVHVGLIIHUHQWZRUGVZHUHUHGXFHGWRWKHVDPHGLVFULPLQDWRU
7KHQRQXQLTXHGLVFULPLQDWRUVFDQEHJURXSHGDVIROORZV•ZLWKRIWKHVHHUURUVDWOHDVWRQHRIWKHZRUGVZDVQRW*HUPDQHJ&HQWDQG&HQWHU

•HUURUVZHUHFDXVHGE\QDPHVHJ$OEUHFKW'UHU VQDPHZDVVWHPPHGWR'XU• HUURUV RFFXUUHG RQ ZRUGV ZLWKRYHUODSSLQJVWHPV ([DPSOHV DUH 5LQG DQG 5LQGH RU
(LVDQG(LVHQ,QRIWKHVHFDVHVWKHHUURUZDVFDXVHGE\VWHPVHQGLQJRQHU

•HUURUVRFFXUUHGRQGHULYHGIRUPVRIWKHVDPHVWHP([DPSOHVIRUWKLVNLQGRIHUURUDUH
7DQ]DQG7lQ]HU

• HUURUV ZHUH FDXVHG E\ 8PODXW VXEVWLWXWLRQ ([DPSOHV DUH 6WXFN DQG 6WFN RU %DU
DQG %lU  PRUH HUURUV ZHUH QRW FDXVHG E\ EXW HQDEOHG E\ 8PODXW VXEVWLWXWLRQ HJ.|USHUDQG.RUSVZHUHERWKVWHPPHGWR.RUS •

DOO UHPDLQLQJ  HUURUV ZHUH FDXVHG E\ SDLUV RI ZRUGV ZKHUH WKH DOJRULWKP VWULSSHG OHWWHUV
IURPERWKZRUGV VWHPV([DPSOHVDUH:LHQDQG:LHVHRU+DIHQDQG+DIHU

7KHVH ILJXUHV DOORZ IRU D FRPSDULVRQ RI FRQWH[WIUHH VXIIL[ VWULSSLQJ ZLWK OLQJXLVWLF DQG GLFWLRQDU\EDVHGVWHPPLQJDOJRULWKPV(UURUVRIWKHILUVWWZRNLQGV 1RQ*HUPDQZRUGVDQGQDPHV FRXOGRFFXUDVZHOOZLWKWKHVHPRUH
VRSKLVWLFDWHGDSSURDFKHV2YHUODSSLQJVWHPVFDQRQO\EHKDQGOHGLIWKHJHQGHURIDZRUGLVNQRZQ'HWHFWLQJ JHQGHU LV YHU\ GLIILFXOW IRU *HUPDQ QRXQV VR PDQ\ RI WKH  HUURUV SURGXFHG E\ VXIIL[VWULSSLQJZRXOGDVZHOOFDXVHSUREOHPVZLWKDQ\RWKHUDOJRULWKP2QO\HUURUVRIWKHODVWWKUHHNLQGVZKLFKDUHDERXWRIDOOQRQXQLTXHGLVFULPLQDWRUVDUHW\SLFDOIRUVXIIL[VWULSSLQJ)RUH[DPSOHWKH3RUWHUVWHPPHUFRQIODWHVJHQHUDOJHQHURXVJHQHUDWLRQDQGJHQHULFWRWKHVDPHURRW>+XOO@$)DVWDQG6LPSOH6WHPPLQJ$OJRULWKPIRU*HUPDQ:RUGV

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WKDQZHUHHYHQVKRUWHUWKDQRUHTXDOWRFKDUDFWHUV7KLVSURRIVWKHDVVXPSWLRQWKDWWKHORQJHU
WKHZRUGVWKHIHZHUDUHWKHHUURUVFDXVHGE\VXIIL[VWULSSLQJ:LWK PRVW VLQJOH GRFXPHQWV VWHPPHG ZLWK WKH DOJRULWKP WKH DPRXQW RI RYHUVWHPPLQJ HUURUV IRU
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GHULYHGIRUPVRIWKHVDPHZRUG2QFHDJDLQWKHJUHDWPDMRULW\RIHUURUFDXVLQJWHUPVZHUHVKRUWHU
WKDQVHYHQFKDUDFWHUV

3.3 Overstemming Verbs, Adjectives, and Nouns
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LI WKHLU ILUVW OHWWHU ZDV QRW RI WKH VDPH FDVH HJ NUDQNHU LV GLVWLQJXLVKDEOH IURP .UDQNHU  $V*HUPDQVHQWHQFHV YHU\VHOGRPVWDUW ZLWK YHUEVRU DGMHFWLYHV WKH DPRXQW RI VDPH ZRUG  GLIIHUHQW
GLVFULPLQDWRU HUURUVRQO\LQFUHDVHGVOLJKWO\)RUWKHPRVWUHSUHVHQWDWLYHWHVWZLWKWKHFRPSOHWH/LQX[ZRUGOLVWWKHIROORZLQJUHVXOWVZHUHREWDLQHG
7KH VL]H RI WKH ZRUGOLVW LV DERXW  WHUPV RI ZKLFK DERXW  DUH QRXQV 7KH VWHPPLQJ
DOJRULWKP FDOFXODWHG  GLIIHUHQW GLVFULPLQDWRUV  RI WKHP IRU QRXQV  7KH QXPEHU RI
RYHUVWHPPLQJHUURUVUDLVHGIURPQRQXQLTXHGLVFULPLQDWRUVIRUQRXQVWRDERXWQRQXQLTXHGLVFULPLQDWRUV   IRU WKH ZKROH ZRUGOLVW $ERXW  ZRUGV ZHUH GHWHFWHG WKDW ZHUH QRW
FRQIODWHG WR DQ XQLTXH GLVFULPLQDWRU  RI DOO ZRUGV  )RU YHUEV DQG DGMHFWLYHV  RI DOO
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