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J a p a n e s e C a s e S t r u c t u r e A n a l y s isb y U n s u p e r v i s e d C o n s t r u c t i o n o f a C a s e F r a m e D i c t i o n a r y

D a i s u k e K a w a h a r a , N o b u h i r o K a ji a nd S a d a o K u r o h a s h iG r a d u a t e S c h o o l o f I n tb r m ~ t ic s , K y o t o U n i v e r s i t y

Yo s h i d a - H o n m a c h i , S ~ k y o -k u , K y o t o , 6 0 6- 8 5 0 1 , J a p a n{ k a w a h a r a , k a j i , k u r o }@ p n c . k u e e . k y o t o - u , a c . j p

A b s t r a c t

I n J a p a n e s e , c a s e s t r u c t u r e a n a l y s i s is v e r y i m -t ) o r t a n t t o h a n d l e s e v e r a l t r o u b l e s o m e c h a r a c -t e r i s t i c s o f J a p a n e s e s n c h a s s c r a m b l i n g , o n f is -

s i on o f e a s e c o m p o n e n t s , m i d d i s a p p e a r a n c e o fc a s e m a r k e r s . H o w e v e r, fi) r l a c k o f a w i d e -c o v e r a g e e a s e f r a m e d i c t i o n a r y, i t h a s b e e n d i f -f i c u lt t o p e r f o r n l c a s e s t r u c t u r e a n a l y s i s a c c u -r at ;e ly. A l t h o u g h s e v e r a l m e t h o d s t o c o n s t r u c ta e a se fl 'm n e d i c t i o n a r y f r o m a n a l y z e d c o r p o r ah a v e b e e n p r o p o s e d , t h e y c a n n o t a v o i d d a t as p a r s e n e s s 1 ) r o l ) l e m . T h i s p a p e r p r o p o s e s a n u n -s u p e r v i se d m e t h o d o f c o n s t r u c t in g a c a se f ra m ed i c t i o n a r y f r o m a n e n o r m o u s r a w c o r p u s b y u s -i n g a r o b u s t a n d a c c u r a t e p a r s e r. I t a l so p r o -r i d e s a c as e s t r u c t u r e a n a l y s i s m e t h o d b a s e d o n

t h e c o n s t r u c t e d d i c t i o n a r y.

1 I n t r o d u c t i o n

S y n t a c t i c a n a l y s is , o r p a r s i n g h a s b e e n a m a i no b j e c t i v e i n N a t u r a l L a n g u a g e P r o c e s s i n g . I nc a s e o f J a t ) a n e s e , h o w e v e r , s y n t a c t i c a n a l y s i sc a n n o t c l a r if y r e l a t io n s b e t w e e n w o r d s i ll s e n -t e n c e s b e c a u s e o f s e v e r a l t r o u b l e s o m e c h a r a c t e r -i s ti c s o f J a p a n e s e s u c h a s s c r a m b l i n g, o m i s s i o no f c a se c o m p o n e n t s , a n d d i s a p p e a r a n c e o f c a s em a r k e r s . T h e r e f o r e , i n J a p a n e s e s e n t e n c e a n a l y -s i s, c a s e s t r u c t u r e a n a l y s i s i s a n i m p o r t a n t i s su e ,a n d a c a s e f r a m e d i c t i o n a r y i s n e c e s s a r y f o r t h ea n a l y s i s .

S o m e r e s e a r c h i n s ti t u t e s h a v e c o n s t r u c t e dJ a p a n e s e c a s e f r m n e d i c t io u a r i e s m a n u a l l y ( Ik e -h a r a e t a l. , 1 99 7 ; I n f b r m a t i o n - Te c h n o l o g y P r o -m o t i o n A g e n c y, J a p a n , 1 9 8 7 ) . H o w e v e r, i t i sq u i t e e x p e n s i v e , o r a l m o s t i m p o s s i b l e t o c o n -s t r u c t a w i d e - c o v e r a g e e a s e f l' an m d i c t i o n a r y b yh a n d .

O t h e r s h a v e t r i e d t o c o n s t r u c t a c a se fl 'm n ed i c t i o n a r y a u t o m a t i c a l l y f r o m a n a l y z e d c o r p o r a

( U t s u r o e t a l. , 1 9 98 ) . H o w e v e r, e x i s t i n g s y n t a c -

t i c a l l y a n a l y z e d c o r p o r a a r e t o o s m a l l t o l e a r n ad i c t i o n a r y, s i n c e c a s e f l' a m e i u f o r m a t i o n c o n s i s t so f r e l a t io n s b e t w e e n n o u n s a n d v e r b s , w h i c h r n ul -t i p l i e s to m i l l i o n s o f c o m b i n a t i o n s .

B a s e d o n s u c h a c o n s i d e r a t i o n , w e t o o k t h ef b ll o w i n g u n s u p e r v i s e d l e a r n i n g s t r a t e g y t o t h e. J a p a ne s e c a s e s t r u c t u r e a n a l y s is :

1 . A t f ir s t , a r o b u s t a n d a c c u r a t e p a r s e r i s d e -v e l o p e d , w h i c h d o e s n o t u t i l i z e a c a s e f l ' m n ed i c t i o n a r y,

2 . a v e r y l a rg e c o r I )u s is p a r s e d b y t h e p a r s e r,

3 . r e l ia b l e n o u n - v e r b r e l a t i o n s a r e e x t r a c t e df r o m t h e p a r s e r e s u l t s , a n d a c a s e f rm n e d i c -t i o n a r y i s c o n s t r u c t e d f r o m t h e m , a n d

4 . t h e d i c t i o n a r y i s u t i l i z e d f o r c a s e s t r u c t u r ea n a l y s i s .

2 C h a r a c t e r i st i c s o f J a p a n e s el a n g u a g e a n d n e c e s s i t y o f c a s es t r u c t u r e a n a l y s i s

I n J a p a n e s e , p o s t p o s i t i o n s f u n c t i o n a s c a s em a r k e r s ( ( M s ) m i d a v e r b i s f i n al i n a s e n t e n c e .T h e b a s i c s t r u c t u r e o f a J a p a n e s e s e n t e n c e i s a sfb l lows :

(1) k a t e 9 a c o a t w o k i~ 'u .

h e n o m i n a t i v e - C M c o a t a c c u s a t iv e - C M w e a r( l i e wears a coa t )

A c l a u s e m o d i f i e r i s l e f t t o t h e m o d i f i e d n o u nas fo l lows :

(2) k a t e 9 a k i t e - i r u c o a tlie n o m - C M w e a r c o a t( t h e c o a t h e w e a r s )

T h e m o d i f i e d n o u n f o l lo w e d b y a p o s t p o s i t i o nt h e n b e c o m e s a c a s e c o m p o n e n t o f a m a t r i x v e r b.T h e t y p i c a l s t r u c t u r e o f a J a p a n e s e c o m p l e x s e n-

t ence i s a s fb l lows :

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(3 ) b o u s h , i n o i r v w a k i t c - i r 'uh a t o f c o l o r t o l ) ic - m a r k e r w e a rc o a t n i a ' w a~ ' c r " ~ , .

c oa l; d a t i v e - C M h a r m o n i z e(c/) h a r m o n i z e s t h e c o l o r o f h i s / h e r h a t w i t ht h e c o a t h e / s h e w e a r s )

I n t e r m s o f a u t o l n a t i c a n a l y s is , t h e p r o b l e n >a t i c c h a r a c t e r i s ti c s o f J a p a n e s e s e n t e n c e s c a n b es u m m a r i z e d a s f o ll o w s:

1. C a s e c o m p o n e n l ; s a r e o f t e n s c r a m b l e d o ro m i t t e d .

2 . C a s e - m a r k i n g p o s t p o s i t i o n s d i s a p p e a r w h e nc a s e c o m p o n e n t s a r e a c c o m p a n i e d b y to p ic -m a r k e r s o r o t h e r s p e c i a l 1 ) o s t p o s i t i o n sm e a n i n g ' j u s t ' , ' a l s o ' a n d o t h e r s .c x ) ka rv 'wa co a t m e t i: it c- iv 'u .

he to l} iC- lna .rke r coa t a l so we ar( I l e wears a coa t a . l so )

3 . A n o u n m o d i f i e d 1 )y a c l a u s e i s u s u a l l y a c a s ec o m p o n e n t f o r t h e v e r b o f t h e m o ( l it ly i ngc l a us e . H o w e v e r , t h e r e i s n o c a s e - m a r k e r f o rt h e i r r e l a t i o n . I n c a s e o f s e n t e n c e 3 , t h e r e i sn o c a s e - m a r k e r f o rc o a t i n r e l a t i o n t o k i t e -i r 'u ' w e a r ' . N o t e t h a t 'h i ( d a t i v e - C M ) o fcoat:n i d o e s n o t s h o w t h e c a s e t o k i t c - i v u ' w e a r ' ,

l i n t to a w a s c r ' v, ' h a r m o n i z e ' .

4 . S e n t e n c e 3 e x h i b i t s a t y p i c a l s t r u c t u r a l a m -1) igui ty in a , l a lmne se sen ten ce . T ha t i s ,ir'o "~va ' c o lo r t o p i c - m a r k e r ' p o s s it ) ly m o d i -t ies k it , c - i r u ' w e a r ' o r a w a . s c v ' u q m r m o n i z e ' .

I n E n g l is h , s e n t e n c e s t r u c t u r e i s r a t h e r r ig i d,a n d w o r d o r d e r ( t h e p o s i t i o n i n r e l a t i o n t o t h ev e r b ) c l e a r l y d e f i n e s c a s e s . I nJapanese,h o w -e w % t h e l ) r o b l em 1 a b o v e m a k e s w o r d o r d e r u s e -l es s, a n d C M s c o n s t i t u t e t h e o n l y i n t' o rm a t i o n f o r

d e t e c t i n g c a s e s .N e v e r t h e l e s s , C M s o f t e n d i s a p l )e a r b e c a u s e o ft h e p r o b l e m s 2 a n d 3 , w h i d l m e a n s t h a t s i m -p l e s y n t a c t i c a n a l y s i s c m m o t c la ri (5 ~ c a s e s s u i [i -c i e n t l > F o r eX a l n p l e , g i v e n a n i n l m t s e n t e n c e :

(4 ) h a r 'c w a D c v , t s c h , - g o m e h a n o , s v,.h e t o p i c - m a r k e r G e r m a l l a l so s l )e a k( h e s p e a k s G e r m a n a ls o )

a s i m p l e s y n t a c t i c a n a l y s i s j u s t d e t e c t s b o t hkar ' c' h e ' a n d D c u t s c h - g o ' G e r m a n ' m o d i f i e s]ta'aas~t' s p e a k ' , b u t t e l ls n o t h i n g a b o u t w h i c h is s u b j e c ta n d o b j e c t . T h i s a n a l y s i s r e s u l t i s n o t s u f fi c i e n t

f o r s u b s e q u e n t N L P a p p l i e a t i o n s l ik e J a p a n e s et o E n g l i s h m a c h i n e l ;r m m l a t io n .

T h e n , w h a t w e n e e d t o d o i s a c a s e s t r u c t u r e .a n a l y s i s b a s e d o n a c a s e f l ' a m e d i c t i o l m r y, o r as u b c a t , o f e a c h v e r b a s fo l lo w s :

h a n a s u ' s p e a k ' :g a (nora ) ks 'r e ' he ' , h i r e ' p e r s o n ''w e ( ace ) c i g o ' E n g l i s h ' , k o t o b a ' l a n g u a g e '~;i?'U,~ % v e a r ' :

g a ( n o r a ) k a v c q m ' ,h i l , o ' p e r s o n 'w e ( a c e ) f u h u ' c l o t h ' ,c o a t ' c o a t 'a ' t l J a s e l ' ~ t ' h a r l l l o n i z e ' :

g a ( n o r a ) kar ' c ' h e ' , h i t o ' p e r s o n ''w e ( a c e ) i r e ~color 'n i ( d a t ) f l t k u ' c l o t h '

C o n s u l t a t i o n o f s u c h a d i c t i o n a r y c a n e a s i l y f i ndt h a t kar 'c ' h e ' i s a n o m i l m t i v e c a s e a n dD e v , t s c h ,-9 0 ' G e r m a n ' i s a n a c c u s a t i v e ( :a s(', i n t h e s e n t e n c e4 .

F u r t h e r m o r e , a (: as e f r a m e d i c t i o n a r yC a l l s o l v et h e p r o b l e m 4 a b o v e , t h a t is , s o m e p a r t o f s t r u c -t u r a l a m b i g u i t y i n s e n t e n c e s . I n c a s e o f s e n t e n c e3, a t ) r ( ) l )er head for' i r 'o w a ' c o l o r t o p i c - m a r k e r '( :a ll })e se l e c ted by c ons u l t in g case s lo t s o fki r 'u~ w e a r ' a n d t h o s e o fa ' w a s c r u ' h a r m o n i z e ' .

3 U n s u p e , r v i s e d c o n s t r u c t i o n o f ac a s e f r a l n e d i c t i o n a r y

T h i s s ( x : t i o n e x p l a i n s h o w t o c o n s t r u c t a c a s efralll(*, d i c t i o n a r y f l ' o m c o r l ) o r a a u t o n m t i c a . l l y.

A s m e n t i o n e d i n t h e i n t r o d u c t i o n s e c t i o n , it;i s q u i t e e x p e n s i v e , o r a h n o s t i l n l) o s s ib l e to c o n -s t r u c t a w i d e - c o v e r a g e c a s e fr a m e d i c t i o n a r y b yl m n d . I n J a p a n e s e ,, s o m e n o u n q - c o p u l a w o r k sl ik e a n a d j e c t i v e . F o r e x a m p l e ,s a ~ t s e i d a ' pos i -t i v e n e s s + C o l m l a ' c a n t a k e9 a c a s e a n d 'h i case .H o w e v e r , s u c h c a s e f r a m e s a r e r a r e l y c o v e r e d t ) y

t h e e x i s t in g h a n d m a d e d i c t i o n a ri e s 1.F m ' t h e r m o r e , e x i s t i n g h a l l d m a d e d i c t i o n a r ie s

c o v e r t y p i c a l o b l i g a t o r y c a s e s l i k eg a ( n o m i n a -t i v e ) , w o ( a c c u s a t i v e ) , n i ( d a t i v e ) , b u t d o n o tc o v er c o m p o u n d c a s e m a r k e r s s u c h a sn i - k a n d z . i t c' i n t e r m s o f ' , ' w o - r n c q u t t e ' c o n c e r n i n g ' a n d o t h -ers .

T h e n , w e tr i e d t o c o n s t r u c t a n e x a m p l e - b a s e dc a s e f r m n e d i c t i o n a r y f r o m c o r p o r a , w h i c h d e -

lOut method collects case frames not only tbr verbs,bu t also tbr adjectives mM nouns-kcopula. In this paper,we use 'ver b' inste ad of 'w;rb/adjective. or llOllll -{- copula.'for s implici ty.

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Ta l / le 2: E X m n l )l e s o f t h e c o n s t r u c t e d e a s e f r a m e s .

v e r l ) st , a S ? t ] g C l " ~ l ,

' h e l p '

y o m ul 'ead '

c a s e l n a r k e l " s

( 1 , o 111 ), , , , o ( i t ( : ( : )' r ~ , i(dat)ae (op). qa (no]n)' w o ( a t ( ; )h i , (dat)& (o 1 0

e x a m p l e n o m l s

husband , pe r son , ch i ld , s t a ff , I , SUSl )ee t , f acu l ty, . . .

. jol) , s hol ) , f a r m w o r k , p r e p a r a t i o n ,e l e c t i o n , move , . . .s o n , f r i e n d , a m b a s s a d o r, m e m l ~ e r, t h a n k , h o l id ~ \y, . ..v o l u n t e ( , r , a ft 'a i r, o t fi c e , r e w a r ( l , h o u s e , h e a d q u a r t e r s , . ..l X ;r s on , ] , c h i h l , a d u l t , p a r e n t , t e a c h e r , . . .n e w s p a p e r , b o o k , m a g a z i n e , a r t i c l e , n o v ( J , l e t t e r , . . .c h i h t, p e r s o n , d a u g h t e r , t e a c h e r , s t u d e n t , r e a d e r , . ..n e w s p a p e r , b o o k , m a g a z i n e , l i b r a ry, c la s s r o o m , b a t h r o o m , ...

r e l ia b l y. T h e r e f o r e , a s fo r f iv e m a j o r a n d t r o u -t ) l e s o m e l i g h t v e r b s ( .~ ' . , r 'u' d o ' , 'nwr'u, q ) c e o m o ? ,ar 'u ' i s . . . ' , iu ~s~w' , na i ' n o t ' ) , t h e i r c a s e f l ' m n e sa r e d i s t i n g u i s h e d d e p e n d i n g ( m t h e ir l e f t n e i g h-b o u r i n g c a s e c o m p o n e n t s . F o r o t h e r ve r b s, w ea S S t l l l l e a ] l n i q u e e a s e f r a m e .

3 . 2 . 2 S t r u c t u r a l a m b i g u i t y

A s s h o w n i n '_ ['~ dfle 1 , K N P d e t e c t s h e a d s o f c a s ec o n l t ~ o n e n t s i n f a M y h i g h a c c u r a c y. H o w e v e r ,i n o r d e r t o c o l l e c t n m c h r e l ia b l e d a t a , w e d is -c a r d e d m o d i t i e r - h c a d r e l a ti o n s in t h e a u l ; on m t i -t a l l y I m r s e d c o r p o r a i n t h e f o l l o w i n g c a s e s :

W h e n C M s o f e a se c o n q x m e n t s d i s a p p e a r

b e c a u s e oi" t o p i c m a r k e r s o r o t h e r s . W h e n t h e v e r b i s f o ll o w e d 1)y a c a u s a t i v e

au x i l i a ry o1' a pas s ive au x i l i a r y, l ;he ca s e tm . t-t ( : r n i s e ] m n g e d a n d t h e 1 ; r a c e i n K N I ' i s n o tso r c l i a l ) l e .

B a s e d o n t h e c o n d i t i o n s a l ) o v e , c a s e c o m p o -n e n t s o f e a c h v e r b a r e c o l l e c t e d f r o l n t h e 1 ) a r s c dc o r p o r a , a n d t h e c o l l e c t e d d a t a a r c c o n s i d e r e d a sc a s e f r a m e s o f v e r b s. H o w e v e r , i f t h e f l c q u e n c yo f a C M is v e r y lo w c o m p a r e dt o o t h e r C M s , i tm i g h t t) e c o l l e c t e d b e c a u s e o f p a r s e e r r o r s . S o ,

w e s e t t h e t h r e s h o l d f o r t h e C M f l e q u e n c y a s2 ~ , w h e r e m . f m e a n s t h e f r e q u e n c y o f t h e1nes t fo l ln ( tChJ. i f t h e f l ' e q u e n e y o f ~ tC M i s lesst l m n t h e t h r e s h o l d , i t is d i s c a r d e d , l.~ br e x a l n p l e ,s u p p o s e t h e m o s t f r e q u e n t C M f in ' a v e r b is w e ,1 0 0 t i m e s , a n d t h e f r e q u e n c yo f n i C M t b r t h ev e r b i s 1.6, n i C M i s d i s c a r d e d ( s i n c e i t i s l e s st h a n t h e t h r e s h o l d , 2 0 ) .

a . 3 C o n s t r u c t e d ca s e f r m n e d i c t i o n a r y

W e a p p l i e d t h e a l )o w ', p r o c e d u r e t o M a i n i c h iN e w s t ) a l ) e r C o r p u s ( 7 y e a r s , 3 ,6 0 0, 00 ( } s e n -

t e n c e s ) . F r o n l t h e c o r t m s , c a s e f r a n w s o f 2 3 , 49 7

v e r b s a r e c o n s t r u c t e d ; t h e a v e ra g e n u m b e r o fe a s e s l o t s o f a v e r b i s 2. 8; t h e a v e r a g e m u n b e r

o f c x a n q f l c n o u n s i n a (:a s(: s l o t i s 3 3 . 6. Ta b l e 2s h o w s e x m n l f l e s o f c o n s t r u c t e d e a s e D a m e s .A l t h o u g h t h e c o n s t r u c t e d d a t a l o o k a p l )r o p ri -

a t e i n m o s t c a s e s , i t i s h a r d t o e v a l u a t e a ( l i c t i o -n a r y s t a t ic a . l l > I n t h e n e x t s e c t i o n , w e u s e t h ed i c t i o m u ' y i n c a se s t r u c t u r e a n a l y s i s a n d e v a l-u a t e t h e a n a l y s i s r e s u l t , w l fi c h a l s o im ] ) li es a nc v a h u ~ . t i ( m o f t h e d i c t i o n a r y i t s e l f .

4 C a s e s t r u c t u r e a n a l y s i s u s i n g t h ec o n s t r u c t e d c a s e f r a m e d i c t i o n a r y

4 .1. M a t c h i n g o f a n i n p u t s e n t e n c e a n d

a c a s e f r a l l l e

' J l~e bas i c 1 ) ro ( : cdu re i n ( ' a se s t ruc l ;u l "e ana lys i s i sl o m a t c h a n i n l m l s e n t e n c e w i t h a c a s e f r a m e ,aS show11 i l l lqgUl 'C, 1 .

T h e m a t c h i n g o f c a s e c on q ) on e n l: s in a n i n p u ta n d c a s e s l o t s i n a c a s e f l ' a ll l O i s ([Olle Oll thef o l l o w i n g c o n d i t i o n s :

I . W h e n a ea s e c o m p o n e n t h a s a C M , i t m u s tb e a s s i g n e d t o 1 ;11o c a s e s l o t w i t h t h e s a m eC M .

. W h e n a c a s e C O m l ) O n e nt d o e s n o l: h a v e aC M , i t c a n 1)e a s s i g n e d t o t h e9 a , w e , o r n iC M s l o t .

. ( ) n l y o n e c a s e c o m p o n e n t c a n b e a s s i g n e dto a c a s e s l o t ( u n i q u e c a s e a s s i g l m m n tc o n -s t r a i n t ) .

T h e c o n d i t i o n s a b o v e m a y p r o d u c e n m l t i l) lem a t c h i n g p a t t e r n s ,a n d t o s e l e c t t h e p r o p e r o n eal l lO ng {, l lc ll l , 11Oll118 of c as e COll lpon( ' , l l tSa l ' o COlll-p a r e d w i t h e x a m p h ' , s i n c a s e s l o t s o f t h e ( ti c ti o -

n a r y.

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syorui wa .(5) document topic-marker /

ka,'e .i .___1!

, , e 1~" walashila" h a n d

[ (1 handed the documen t to him.)

WaRlSU ' hand '

ga defendam, president . . .

we mone y, nlelllO, bribe ...

ni person, suspect, ...

de affair, office, room ...

(6 ) Deutsch-go me/ Gcl'nlan also q

h H I I ( I S I I

s p e a k " - 7

{';cq/;i'i;ir

a teacher who speaks also German)

~ professor, president 1

n i pers on , f ri end . . .

- - w e re as on E ng lish J ap a ne se

to ( sen tence)

Figure 1: Matc hin g of an inl)ut se ntenc e and a case fl:ame.

Even though a 3,600,000 sentences corpus wasused for learning, examples in case slots are stillsparse, and a n input noun most ly does not matchexactly an example in the dictionary. Then, athesaurus is employed to solve this problem.

In our experiments, NTT Semantic FeatureDictionary (Ikehara et al., 1997) is employed asa thesaurus. Suppose we calculate the silnilar-ity between Wl and w2, their depth is dl and d2in the thesaurus, and the d epth of their lowest(most specitic) common node is de, the similarityscore between them is calculated as follows:

= ( 4 +

If W1 and w2 are in the same node of the the-saurus, the similarity is 1.0, the maximum scorebased on th is criteria. If Wl and w2 are identical,the similarity is 1.0, of course.

The score of case assigmnent is the best sim-ilarity between the input noun and examples inthe case slots. The score of a match ing pat ter n

is the sum of scores of case assi gnme nts in it. Iftwo or more patterns meet the above conditions,one which has the best score is selected as a finalresult.

In the case of sentence 5 in Figure 1, karc 7ti'he dativc-CM' is assigned to the ni case slot.Then, s y o r u i w a 'document topic-marker ' can beassigned to the g a or w o case slot. By calculatingsimilarity between s y o r u i and 9a-slot examplesand wo-slot exmnples, it; is considered to be as-signed to the wo slot.

In case of sentence 6, none of the case compo-

nents has a CM. Based on similarity calculation,

Deu t sch , -go is assigned to 'wo, s e n s e i is assignedto ga.

4 . 2 P a r s i n g w i t h c a s e s t r u c t u r e a n a l y s i s

A complex sentence which contains a clausalmodifier exhit)its a typical structural ambiguityof Japanese; case components left to a verb ofa c lau sal mo difi er, Vc, possi bly mo di fy V~: or amatrix verb Vm.

For example, in sentence 3, ir 'o 'w~L 'colortopic-inarker' possibly modifies k i t e - i ru 'wear' or(l,~l)(],Sel'~l, q l a r i i l o n i z e ' .

KNP, a rule-based parser, handles this type ofambiguity ~s follows. If a case component is fol-lowed by a com ma, it is tre ate d as modif[ying Vm ;if not, it is tre ate d as modif[ying 1~:. Alt hou ghthis heuristic rule usually explains real data verywell, sentence 3 will be analyzed incorrectly.

Parsing which utilizes a case frame dictionarycan consider which is a proper head, V~ or Vm,tbr an ambiguous case compolmnt by comparing

examples in the case slots of V~ and 14~. Sucha consideration nmst be done considering wlmtother case components modifly Vc and Vm, sincethe assigned case slot of a case component mightdiffer depending on the candidate structure ofthe sentence due to the unique case assignmentconstraint.

Therefore, it is necessary to expand the struc-tural ambiguity and consider all the possiblestru cture s fbr an input. So, we calculate thematching score of all pairs of case componentsand verbs in all possible stru cture s of the sen-

tence, and select the best structure based on the

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boushi t i c ; ir e w a

h a t c o l o r

bott.~]tin oh a l ~ , ~

~ - - - _ _ i r e w a

c o l o r - - I

C ( ; l tIt[

C O a t ~5 " r l lharl l lOll iZe

W e C l O t h , t l ll i ]' /) l 'l l l, C I ) l a . . . ~ W O ] p O W C I ' ,face, } l} ind

~ . II de party, oily, home ... ni I prcl)lc nce, cItlth ....

kite-it 'lt Co(It I1i H W C LT C I ' l l

w e a l " c o a t h l l r n l o ll i Z C

ha ti r e w a - 2 ( d i s t a n c e p e n a l t y )c l r ~ ]

ki te - i ru [

co . t , ,i

~ COat IdilA't!rll

k i r u ' w e a r ' ~.\ awa,~emt 'harllloilizc'

2 ,:2,,,,,,6...

Figure 2: Parsing with east structure analysis.

sum of the matching scores in it.

Since the heuristic rule employed ill KNP isactually very useful, we in(:orporate it, that is,l)enalty score is imposed to the modifier-hea(l re-la.tion dep end ing on t he distr aint betwee n ~t mod -ifi(;l" an d a hea d. If a moditi er is not followed bya comma, the penalty score, 0, -2 ,- 4, -6 , . .. isimp ose d w hen a modi tler modifie.s the first (nea.r-est), second, third, tburth, ... verbs ill a sentencerespectively; if with a comma, th e tmna lty score,-2 , 0, -2 , -4 , ... is impose&

For example, sentence 3 was analyzed t)y ourmethod a s shown ill Figure 2. Since the simi-

lm'ity score between fro ~color' a.nd the 'we-slotof uwa.s'cr'u hmun oniz e is nmc h lar ger t;]iall theftl)etween i re 'eoloff and th e ga-slot of lci'r'u. 'wear',the correct str uctu re of the selltellee was de-tected (the right-lmnd parse of Figure 2) . Notethat, furthermore, both the ease of i r e ill reb>tion to a w a s c r u 'harmonize' , and the case of coal,in relation to k i t e - i r u 'wear' were dete(:ted cor-rectly.

Structm'al ambiguities often cause a combina-toria l explosio n when a sente nce is long. How-ever, by de tec tin g the SeOl)eS of coord ina te st ruc-

tures 1)e%rehand, which off;ell aPl)ear in long

'l'td)le 3: The at:curacy of case detect ion.

(;orre(:I; ill(:orl'e(;t])arsing

ease casee r r o r

(lel;e(:l;ion (tete(:tiontopic- marle er 82 13 5

clausal modif ier 73 18 9

senl;ences, we can reasonably limit the possil)lesl;ructures of the sentence.

The ~werage analysis speed of tile ext)criments

described in the next section was about 50 sen-ten ets /ar ia. 'File tinm-oul, of one rain. was onlyemployed to 7 out of 4,272 test Selltellces.

4 . 3 E x p e r i l n e n t s a n d d i s c u s s i o n

W e u se d 4 , 2 7 2 s e n t e n c e s o f K y o t o U n i v e r s i ty c o lpu s as a test set. We parsed them by our newlnethod (Figure 3 shows several examt)les) andcheekc, d two 1)oints: case d etec tion of mnb iguo uscase (-omponents and syntactic analysis.

First, we randomly selected ambiguous easecomp onen ts: 100 l,ol)ic-markcA case comp on ent s

all(t 100 (:ase coral)orients moditied by clausal

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ookll rasyo ha

the Treasury

3gatsuki kes:~an desettlenlellt ill Marc h ,,

.l'hintakuginkotl kak ukm t ga I impr