np vp cost = 0 cost = c np vp cost = c I replacing term cost = c+c n cost = c * Error detection Error correction pron det pron det n gi

Transcription

1 Spoken Language Paring with Robutne and ncrementality Yohihide Kato, Shigeki Matubara, Katuhiko Toyama and Yauyohi nagaki y Graduate School of Engineering, Nagoya Univerity y Faculty of Language and Culture, Nagoya Univerity z Center for ntegrated Acoutic nformation Reearch, Nagoya Univerity x Furo-cho, Chikua-ku, Nagoya, Japan yoihide@inagaki.nuie.nagoya-u.ac.jp Abtract Thi paper decribe a method of incremental paring for grammatically ill-formed entence. The method conit of incremental chart paring and error correcting, which are executed ynchronouly. The error correction at an early tage enable the parer to contruct a yntactic tructure of an initial fragment of a grammatically ill-formed entence whenever a word i inputed. The parer ha two feature: One i to utilize the yntactic contraint uch a reachability and connectability, and the other i to aign a cot, which repreent degree of error correction, to every yntactic tructure. Therefore, the parer, at any time, adopt the yntactic tructure of the well-formed entence which i mot imilar to the input entence a a paring reult. ntroduction t i important fora peech dialogue ytem to interact with a uer naturally and moothly. Suchytem i required to poe the capability of immediate repone to the uer, o that the uer can recognize the degree of the ytem' undertanding at any time. To atify thi requirement, the pontaneou peech dialogue ytem hould be able to interpret poken language at leat incrementally (Milward and Cooper, 994; nagaki and Matubara, 995). ncremental interpretation mean to proce an input entence from left to right and extract emantic information from the initial fragment ofit. Another problem on the pontaneou peech dialogue ytem i whether it can deal with grammatical ill-formedne ince grammatically ill-formed expreion appear frequently in poken language. Mot of the ytem which have been developed o far, however, aume that input entence are grammatically well-formed. To olve the problem, ome technique for proceing grammatically ill-formed entence have been propoed. For example, unication-baed framework (Dougla and Dale, 99; maichi and Matumoto, 995) deal with the ill-formedne uch a contraint violation. Partial paring (Jenen et al., 983; McDonald, 99) extract the information from the partial tructure contructed in the paring proce. Extended paring algorithm (Saito and Tomita, 988; Mellih, 989; Kato, 994; Lee et al., 995) deal with the yntactic ill-formedne uch a deletion, inertion and mutation of word. Little attention, however, ha beeven to the viewpoint of incrementality in thee literature. Thi paper decribe a method of incremental paring for grammatically ill-formed entence. The method conit of incremental chart paring (Matubara et al., 997) and error correcting, which are executed ynchronouly. The error correcting, at any time, procee yntactic error uch a deletion, inertion and mutation of word. The error correction at an early tage enable the parer to contruct ayntactic tructure of an initial fragment of a grammatically ill-formed entence whenever a word i inputed. The parer ha two feature: the rt one i to utilize the yntactic contraint uch a reachability and connectability o that it can correct the error eciently. The econd one i to aign a cot, which repreent degree of error correction, to every yntactic tructure. The parer adopt the yntactic tructure of the well-formed entence which i mot imilar to the input entence a a paring reult. n addition, in order to increae eciency of paring, the parer delete the tructure which do not contribute to the contruction of the complete tructure with a minimum cot. Thi paper i organized a follow: The next ection i a ummary of incremental chart paring. n ection 3, we decribe the error correcting method. Report of an eciency evaluation of our paring ytem are given in ection 4. n ection 5, we make a comparion with related work. ncremental Chart Paring n thi ection, we ummarize incremental chart paring (Matubara et al., 997). The incremental chart paring record paring reult in a graph called a chart. The edge i labeled a yntactic tructure called a term which i denoted an expreion [] x, where x i a category and i a lit of term, a word or a pecial ymbol `?'. The term [?] x i called an undecided term. The leftmot

2 input: w w wn output: chart chart := f(; ; [?])g; for i =to n begin )conulting dictionary: if category of wi i x then add (i ;i;[wi]x) tochart )applying grammar rule: for each (i ;i;[]x ) chart and each grammar rule a! x y z if x = x then add (i ;i;[[]x [?]y [?]z]a) tochart 3)replacing term: for each (;i ;); (i ;i;[]x) chart if the category of lut() i x then add (;i;rep(; []x)) to chart end Figure : Algorithm of incremental chart paring! vp! vt pron! vt! np! pron vp! vi pp det! the vi! np! det n vp! be adj n! train be! i np! gi pp pp! pnp gi! going adj! bet np! n p! Figure : Example of Grammar occurrence of an undecided term in a term i called a leftmot undecided term in, which i denoted lut(). f an edge i labeled the term which include an undecided term, it i aid to be active, otherwie inactive. The incremental paring algorithm i hown in Figure. Here, the edge with label between the node j and k i denoted (j; k; ). The reult of replacing the leftmot undecided term in with i denoted rep(;). The incremental chart paring introduce two new operation to the tandard bottom-up chart paring (Kay, 98). One i the application of grammar rule to an active edge and the other i the replacement of the leftmot undecided term with the term of an activeedge. The operation enable the parer to contruct a yntactic tructure of the initial fragment of an input entence. A an example, let u conider the pare of the following entence uing the grammar hown in Figure : going train i bet. (.) When \" i inputed after the input of \", the tandard bottom-up chart parer contruct term [[[] pron ] np [?] vp ],[[] vi [?] pp ] vp and [[] vt [?] ] vp. On the other hand, the incremental chart parer alo contruct term: [[[] pron ] np [[] vt [?] ] vp ] (.) [[[] pron ] np [[] vi [?] pp ] vp ] (.3) Thee term, at leat, repreent the relation that \" i the ubject of the verb \" which can be acquired from the initial fragment \ ". Thi how that the incremental chart paring ha capability of contructing a yntactic tructure of an initial fragment at an early tage. 3 ncremental Paring of ll-formed Sentence Thi tudy deal with three kind of yntactic error uch a deletion, inertion and mutation of word. n thi ection, we extend the incremental chart paring to proce grammatically ill-formed entence which include error with keeping high degree of incrementality. 3. Structure Contruction for ll-formed Sentence Our purpoe i to develop a method of contructing a yntactic tructure of the initial fragment of the entence including error at any time. Thi i accomplihed eaily the following operation: inerting aword (for a deletion error), kipping a word (for an inertion error) and replacing a word with another one (for a mutation error). Let u conider the following entence: train i bet. (3.) t might be upected that (3.) include one of the following error:. deletion of \going" in the entence \ going train i bet.". inertion of \" in the entence \ train i bet." 3. mutation of \the" into \" in the entence \ the train i bet." n the rt cae, the error can be corrected inerting a gerund \going". Actually, if the parer add the edge labeled the term [going] gi to the chart and applie grammar rule to it, the parer contruct a term (3.). [[[going] gi [?] pp ] np [?] vp ] (3.) The parer replace the leftmot undecided term [?] in (.) with (3.) o that contruct a term [[[] pron ] np [[] vt [[[going] gi [?] pp ] np [?] vp ] ] vp ] for the initial fragment \ ". n the econd cae, the error can be corrected kipping \" o that the parer contruct a term [[[] pron ] np [[] vt [[[train] n ] np [?] vp ] ] vp ] for the initial fragment \ train". Finally, in the third cae, the error can be corrected replacing \" with a determiner \the". Then, the parer contruct a term: [[[] pron ] np [[] vt [[[the] det [?] n ] np [?] vp ] ] vp ]

3 The above example how that the incrementalchart parer can contruct yntactic tructure of the initial fragment of ill-formed entence the above three operation at an early tage. 3. Error Detection The previou ection ha illutrated the tructure contruction for a grammatically ill-formed entence. The operation, however, donotalway enable the parer to contruct a yntactic tructure. For example, if \train" i kipped in (3.), no yntactic tructure can be contructed. From the viewpoint of eciency, the parer hould not execute uch operation. Thu, error detecting technique i required. What i important i that error detecting hould be executed a oon a poible. For thi purpoe, the parer detect error utilizing the information included in the paring reult and alookahead word. Since uch information can be extracted eaily from the initial fragment which ha been already inputed, high degree of incrementality can be achieved. 3.. Reachability and Connectablity To formalize the information which i ued for the error detection, we dene two relation between yntactic categorie: reachability and connectability. Let and be any equence of categorie and let a, x, y and z be any categorie. Denition(Reachability) x i reachable to y if x and y atify one of the following condition:. x = y.. A grammar rule y! x exit. 3. For ome z, x i reachable to z and z i reachable to y. The relation that x i reachable to y mean that a tructure whoe root i y ha the leftmot decendant x. We writex ; y if x i reachable to y. Denition(connectability) x i connectable to y if x and y atify the following condition. or.:. A grammar rule a! xz exit and y ; z.. A grammar rule a! x exit and a i connectable to y. The relation that x i connectable to y mean that a category equence xy i allowed the grammar. We writex _ y if x i connectable to y. 3.. Error Correction Procedure n our method, the parer correct error whenever a word i inputed. The procedure for error correction are hown in Figure 3. Here, we ue a pecial ymbol `3' for the reult of inertion or replacement of a word error correction. The reachability relation i utilized for the error detecting uing the information included in the paring reult. The connectability relation i utilized for the error detecting uing a lookahead word. deletion correction: for each (;i ;); (i ;i;) chart and each category x if x ; and x _ then add (i ;i ; [3]x) tochart inertion correction: for each (;i ;) chart and each category x of wi if x ; then add (i ;i ;")tochart mutation correction: for each (;i ;); (i ;i;) chart and each category x if x ; and x _ then add (i ;i ; [3]x) tochart Figure 3: Procedure for error correction For example, a Figure 4 (a), let u conider deletion correction for the entence (3.) when \"i inputed. The categorie of the leftmot undecided term in the term (.) and (.3) are and pp, repectively. The categorie which are reachable to or pp are gi, n, p and pron. Moreover, the only gi in thee categorie i connectable to the category p of \". Therefore, the parer inert a word whoe category i gi. The parer correct the other error in the ame way a Figure 4 (b) and (c) how. 3.3 Cot of Error Correction n order to keep high degree of incrementality, the parer execute the error correcting at any time. Thu, it may execute the error correcting where error do not occur actually and contruct yntactic tructure of the entence which i not imilar to the input entence. Such tructure hould be eliminated from the paring reult. For thi purpoe, the cot which repreent degree of error correction i aigned to every yntactic tructure and the parer adopt the tructure with a minimum cot a a paring reult. The calculation of the cot value i a follow:. Aigning a cot to every error correction.. f the parer contruct a tructure error correction, add the cot of the error correction to the cot of the tructure. For example, when \" i inputed, a term [[[] pron ] np [?] vp ] (3.3) i contructed in the paring phae. \" i inputed, a term Next, when [[[3] n ] np [?] vp ] (3.4) i contructed in the error correcting phae. The cot value of the term (3.3) i le than that of (3.4) ince (3.3) i with no error correction and (3.4) i with mutation error correction. Conequently, the parer adopt (3.3) a a paring reult. From the viewpointofeciency, the error correcting may increae the number of edge in the chart o that the paring become inecient. To improve eciency of the paring, the parer hould delete the

4 Error correction (a) Deletion correction np pron vi vp Reachability vt vi p adj be Error detection Connectablity p Edge gi (b) Mutation correction pron vi 3 vt vi p adj be 3 n 3 det np vp 3 4 train 3 train 4 3 train 4 (c) nertion correction Reachability pron vi Connectability n 3 4 train Active edge nactive edge edge error correction 3 train 4 Figure 4: Example of error correction edge deletion: for each (;i;); (;i;) chart if c() =c( ) andcot() < cot( ) then delete (;i;) from chart Figure 5: Procedure of edge deletion : cot = cot = c pron pron replacing term cot = c : cot = c n cot = c+c n cot = c * * Figure 6: Comparion between the cot of yntactic tructure tructure which do not contribute to the contruction of the complete tructure with a minimum cot. Therefore, the parer execute edge deletion hown in Figure 5. Here, the cot value of a term i denoted cot() and a equence of categorie of the undecided term which occur in a term i denoted c(). Let u conider an example of edge deletion. The term in Figure 6 i contructed when \" i inputed. The term in Figure 6 i contructed mutation correction when \" i inputed. c() and c() are vp. We know cot() < cot() ince i with no error correction and i with mutation correction. Thu, the edge (; ; ) i deleted. 3.4 An Example of Paring Thi ectiove an example of incremental paring of grammatically ill-formed entence. Table how the chart contruction proce for the entence (3.). We aume that the cot value of every error correction i. When \" i inputed, a gerund i inerted deletion correction and the edge #6 i contructed. When \train" i inputed, \" i kipped inertion correction and the edge # i contructed. Next, \" i replaced with a determiner mutation correction and the edge # i contructed. The edge #3, #, #5, #8 and #9, which are contructed in error correcting phae, are deleted edge deletion. The example illutrate that the parer can contruct yntactic tructure of initial fragment of the grammatically ill-formed entence (3.) whenever a word i inputed. The edge which are contructed nally are #3, #4 and #5, and each edge i contructed deletion, inertion or mutation correction, repectively. 4 Evaluation of Eciency We have made an experiment toevaluate eciency of the method propoed in thi paper. The experiment ha been made under the following condition: The parer ha been implemented in GNU Common Lip on UltraSPARC- (Main memory 5MB, CPU 48MHz) The grammar conit of 74 rule and 54 categorie. The cot value of any error correction i regardle of the kind of an error. 6 grammatically ill-formed entence are generated at random in the following way: Firt, well-formed entence are generated at random

5 Table : ncremental paring proce of \ train i bet." chart edge word # loc term cot deletion - [?] - [[[]pron]np[?]vp] 3 - [[[3]n]np[?]vp] deleted 4 - [[[]pron]np[[]vt[?]]vp] 5 - [[[]pron]np[[]vi[?]pp]vp] 6 - [[[]pron]np[[]vt[[[3]gi[[?]pp]np[?]vp]]vp] 7-3 [[[]pron]np[[]vi[[]p[?]np]pp]vp] 8-3 [[[]pron]np[[]vt[[[3]gi[[]p[?]np]pp]np[?]vp]]vp] train 9-3 [[[]pron]np[[]vi[[]p[[3]det[?]n]np]pp]vp] -3 [[[]pron]np[[]vt[[[3]gi[[]p[[3]det[?]n]np]pp]np[?]vp]]vp] deleted -3 [[[]pron]np[[]vt[?]]vp] -3 [[[]pron]np[[]vt[[[3]det[?]n]np[?]vp]]vp] 3-4 [[[]pron]np[[]vi[[]p[[train]n]np]pp]vp] 4-4 [[[]pron]np[[]vt[[[3]gi[[]p[[train]n]np]pp]np[?]vp]]vp] 5-4 [[[]pron]np[[]vi[[]p[[3]det[train]n]np]pp]vp] deleted 6-4 [[[]pron]np[[]vt[[[train]n]np[?]vp]]vp] 7-4 [[[]pron]np[[]vt[[[3]det[train]n]np[?]vp]]]vp] i 8-4 [[[]pron]np[[]vi[[]p[[[3]pron]np]pp]vp] deleted 9-4 [[[]pron]np[[]vt[[[3]gi[[]p[[3]pron]np]pp]np[?]vp]]]vp] deleted -5 [[[]pron]np[[]vt[[[3]gi[[]p[[train]n]np]pp]np[[i]be[?]adj]vp]]]vp] -5 [[[]pron]np[[]vt[[[train]n]np[[i]be[?]adj]vp]]]vp] -5 [[[]pron]np[[]vt[[[3]det[train]n]np[[i]be[?]adj]vp]]vp] bet 3-6 [[[]pron]np[[]vt[[[3]gi[[]p[[train]n]np]pp]np[[i]be[bet]adj]vp]]vp] 4-6 [[[]pron]np[[]vt[[[train]n]np[[i]be[bet]adj]vp]]vp] 5-6 [[[]pron]np[[]vt[[[3]det[train]n]np[[i]be[bet]adj]vp]]vp] from the grammar (the length of the entence i4,6,8orandthenumber of the entence i 5 for each length). Second, random occurrence of pecic type of error are introduced into thee entence. We meaured the wort proceing time per word in the following cae: the paring with both error detection and edge deletion the paring without error detection and with edge deletion the paring with error detection and without edge deletion Table how average of the wort proceing time per word. The time in the paring without edge deletion wa a great deal longer than that with edge deletion. The time in the paring without error detection wa longer than that with error detection. Thi reult demontrate that our propoed method i ueful for increaing eciency of the paring. 5 Related Work Firt, we compare our work with related work from the viewpoint of incrementality, i.e., the timing of error correcting and the contruction of yntactic tructure on the way. Mellih (Mellih, 989) and Kato (Kato, 994) have propoed a method baed on chart paring. The method i divided into the bottom-up phae and the top-down phae. The bottom-up phae contruct tructure of an input entence in the bottom-up chart paring manner. f it failed to contruct a complete tructure, the top-down phae i tarted to detect error. The bottom-up chart paring cannot contruct a tructure of the initial fragment ofaentence and, conequently, Mellih' and Kato' parer alo cannot contruct it. n addition, it i not until the bottom-up phae i nihed that the error detection proce i invoked. Moreover, error correcting need all of the partial yntactic tructure contructed in the bottom-up chart paring. There, the method cannot execute paring and error correcting ynchronouly. Saito' method i on bai of Generalized LR paring (Saito and Tomita, 988). The method execute paring and error correcting ynchronouly a our. Saito' parer, however, cannot contruct a tructure of the initial fragment ofaentence, becaue the method i baed on Generalized LR parer. Lee extended Earley Algorithm to deal with grammatically ill-formed entence (Lee et al., 995). Their parer perform paring and error correcting ynchronouly. The algorithm cannot contruct a yntactic tructure while paring i performed. On the other hand, our propoed parer can proce grammatically ill-formed entence incrementally ince it execute paring and error correcting ynchronouly. n addition, the parer can contruct

6 Table : Wort proceing time per word Kind of Length of with both error detection without error detection with error detection Error Sentence and edge deletion and with edge deletion and without edge deletion 5.8 ec.48 ec.44 ec inertion of 7.58 ec.37 ec.548 ec aword 9.39 ec.6 ec.964 ec.4 ec.8 ec ec 4.64 ec.38 ec.6 ec mutation of 6.6 ec.34 ec.3 ec aword 8.6 ec.83 ec 4.54 ec.39 ec.3 ec 56. ec 3.8 ec.88 ec.4 ec deletion of 5.7 ec.4 ec.46 ec aword ec.4 ec.6 ec ec.34 ec 6.96 ec tructure of the initial fragment of grammatically ill-formed entence ince it i baed on the incremental chart paring. Next, our work i compared with Saito' and Lee' from the viewpoint of eciency of the error detecting. Saito' parer utilize LR paring table which predict what word i the next input in order to detect error. n Lee' method, the parer predict the next contituent utilizing the paring reult. Saito' and Lee' error detecting are identical to the error detecting which utilize the reachability relation. On the other hand, the error detecting whichhave been propoed in thi paper i more ecient ince the error detecting utilize not only the reachability relation but alo the connectability relation. 6 Concluion Thi paper ha propoed an incremental and robut paring method for grammatically ill-formed entence. The method execute paring and error correcting ynchronouly. Correcting error at any time enable the parer to contruct yntactic tructure of the initial fragment of grammatically ill-formed entence whenever a word i inputed. Uing our method, the number of the tructure may increae exploively becaue the parer contruct them not only in paring phae but alo in error correcting phae. n order to decreae it, a cot ha been aigned to every error correction. n future work, we intend to apply the method propoed in thi paper to the incremental poken language tranlation ytem (Matubara and nagaki, 997). Reference S. Dougla and R. Dale. 99. Toward Robut PATR. n Proc. of COLNG-9, page 468{474. O. maichi and Y. Matumoto ntegration of Syntactic, Semantic and Contextual nformation in Proceing Grammatically ll-formed nput. n Proc. of JCA-95, page 435{44. Y. nagaki and S. Matubara Model for ncremental nterpretation of Natural Language. n Proc. of SNLP-95, page 5{6. K. Jenen, G. E. Heidorn, L. A. Miller, and Y. Ravin Pare Fitting and Proe Fixing: Getting a Hold on ll-formedne. Computational Linguitic, 9(3{4):47{6. T. Kato Yet Another Chart-Baed Technique for Paring ll-formed nput. n Proc. of ANLP- 94, page 7{. M Kay. 98. Algorithm Schemata and Data Structure in Syntactic Proceing. n Technical Report CSL-8-. K.J. Lee, J.K. Kweon, J. Seo, and G.C. Kim A Robut Parer Baed on Syntactic nformation. n Proc. of EACL-95, page 3{8. S. Matubara and Y. nagaki ncremental Tranfer in Englih-Japaneae Machine Tranlation. ECE Tran. on nf. & Sy, E8- D():{9. S. Matubara, S. Aai, K. Toyama, and Y. nagaki Chart-baed Paring and Tranfer in ncremental Spoken Language Tranlation. n Proc. of NLPRS-97, page 5{54. D.D. McDonald. 99. An Ecient Chart-baed Algorithm for Partial-Paring of Unretricted Text. n Proc. of ANLP-9, page 93{. C.S. Mellih Some Chart-Baed Technique for Paring ll-formed nput. n Proc. of ACL-89, page {9. D. Milward and R. Cooper ncremental nterpretation : Application, Theory, and Relationhip to Dynamic Semantic. n Proc. of COLNG- 94, page 748{754. H. Saito and M. Tomita Paring Noizy Sentence. n Proc. of COLNG-88, page 56{566.