{ "paper_id": "O01-1011", "header": { "generated_with": "S2ORC 1.0.0", "date_generated": "2023-01-19T08:09:33.691332Z" }, "title": "An Empirical Study of Zero Anaphora Resolution in Chinese Based on Centering Model", "authors": [ { "first": "Ching-Long", "middle": [], "last": "Yeh", "suffix": "", "affiliation": { "laboratory": "", "institution": "Tatung University", "location": { "addrLine": "40 Chungshan N. Rd. 3 rd . Section Taipei 104 R.O.C" } }, "email": "" }, { "first": "Yi-Jun", "middle": [], "last": "Chen", "suffix": "", "affiliation": { "laboratory": "", "institution": "Tatung University", "location": { "addrLine": "40 Chungshan N. Rd. 3 rd . Section Taipei 104 R.O.C" } }, "email": "" } ], "year": "", "venue": null, "identifiers": {}, "abstract": "In this paper, we describe the creation of Chinese zero anaphora resolution rules by performing experiments. The rules were constructed based on the centering model. In the experiments, we selected several texts as testing examples. We compared the referents of zero anaphors in the testing texts identified by hand with the ones resolved by using an algorithm employing a resolution rule. Three rules were used to carry out the experiment. The results show that the rule considering grammatical role criteria and domain knowledge obtained the best result: 85% of zero anaphors in the test texts were correctly resolved. We investigate problems of miss-resolution of zero anaphors in the test text and propose solution to deal with them.", "pdf_parse": { "paper_id": "O01-1011", "_pdf_hash": "", "abstract": [ { "text": "In this paper, we describe the creation of Chinese zero anaphora resolution rules by performing experiments. The rules were constructed based on the centering model. In the experiments, we selected several texts as testing examples. We compared the referents of zero anaphors in the testing texts identified by hand with the ones resolved by using an algorithm employing a resolution rule. Three rules were used to carry out the experiment. The results show that the rule considering grammatical role criteria and domain knowledge obtained the best result: 85% of zero anaphors in the test texts were correctly resolved. We investigate problems of miss-resolution of zero anaphors in the test text and propose solution to deal with them.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Abstract", "sec_num": null } ], "body_text": [ { "text": "In Chinese text, anaphors are frequently eliminated, termed zero anaphor (ZA) hereafter, due to their prominence in discourse [LT81] . For example in (1), the topic of the utterance (1a) is 'Electronics stocks,' which is eliminated in the second utterance and the topic of utterance (1c), 'Securities stocks,' is eliminated in the utterance (1d).", "cite_spans": [ { "start": 73, "end": 77, "text": "(ZA)", "ref_id": null }, { "start": 126, "end": 132, "text": "[LT81]", "ref_id": "BIBREF14" } ], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "(1) a. A simple rule, Rule 1, can be formulated by observing the phenomenon of topic chain in Chinese text. This rule can be used to correctly resolve the referent of the ZA in (1a), for example.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "Rule 1: If a ZA occurs in the topic position of utterance i, then its antecedent is the topic of utterance i-1.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "In general, zero anaphors in Chinese can occur in any grammatical slot with an antecedent that may occur in any grammatical slot, regardless of their distance [LT79] .", "cite_spans": [ { "start": 159, "end": 165, "text": "[LT79]", "ref_id": "BIBREF13" } ], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "Thus Rule 1 is obviously insufficient to account for the resolution of ZAs.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "Within the theories of discourse, Centering is a computational model, which has been developed as a methodology for the explanation of the local coherence and its relationship to attentional state at the local level and focuses on pronominal and nominal anaphora [GJW83, GJW95] . It is formalized as a system of constraints and rules, which can, as part of a computational discourse model, act to control inference [JW81] . In the centering model, each utterance in a discourse segment has two structures associated with it, called Forward-Looking and Backward-Looking centers,", "cite_spans": [ { "start": 263, "end": 270, "text": "[GJW83,", "ref_id": "BIBREF2" }, { "start": 271, "end": 277, "text": "GJW95]", "ref_id": "BIBREF4" }, { "start": 415, "end": 421, "text": "[JW81]", "ref_id": "BIBREF11" } ], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "which correspond approximately to Sidner's potential foci and discourse focus [Sid79] .", "cite_spans": [ { "start": 78, "end": 85, "text": "[Sid79]", "ref_id": "BIBREF18" } ], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "Forward-Looking Centers, C f , is a set of discourse entities in an utterance, and", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "Backward-Looking Center, C b , is a special member of this set, which is the discourse entity that the utterance most centrally concerns. Our analysis is based on this computational model to resolve the intersentential ZAs.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "In this paper, we aim at formulating rules for the resolution of zero anaphors in Chinese. We start with a rule, Rule 2, formulated by employing the centering model.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1." }, { "text": "For each utterance U i in a discourse segment U 1 , \u2026 , U m : If C b (U i )", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rule 2:", "sec_num": null }, { "text": "is realized by a zero anaphor in U i+1 then the C b (U i+1 ) must be realized by", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rule 2:", "sec_num": null }, { "text": "C b (U i ).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rule 2:", "sec_num": null }, { "text": "We performed an experiment by using an algorithm employing this rule to see how the ZAs in news text are resolved. The initial result showed that about half of the ZAs could not be correctly resolved. Consequently we considered adding other constraints, such as grammatical role criteria and semantic knowledge, to enhance the rule and get better results. We repeated the experiment and the result showed that about 85% can be correctly resolved by using the new rule. The remaining 15% errors of the ZAs resolution occur because of the lack of sufficient semantic knowledge and the character of locality of centering model. We further investigate these situations and", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rule 2:", "sec_num": null }, { "text": "propose an approach to solve the problem.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rule 2:", "sec_num": null }, { "text": "In the next section we describe t his nature of zero anaphora in Chinese. In Section 3, w e describe the centering model, and we illustrate the result of the empirical study we made by observing the industry news we collected in Section 4.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rule 2:", "sec_num": null }, { "text": "The discussion and implementation are in Section 5 and 6, respectively, and finally conclusions and future works are made.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rule 2:", "sec_num": null }, { "text": "In Chinese, anaphors can be classified as zero, pronominal and nominal forms, as exemplified in (2) by i , and , respectively [Chen87] 1 . Zero anaphors are generally noun phrases that are understood from the context and do not need to be specified.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Zero Anaphora in Chinese", "sec_num": "2." }, { "text": "(2) a.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Zero Anaphora in Chinese", "sec_num": "2." }, { "text": "i Zhangsan frightened and ran outside. (3) The house, (someone) has finished building it.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Zero Anaphora in Chinese", "sec_num": "2." }, { "text": "In the intersentential case, antecedent and anaphors are located in different sentences. Depending upon the distance between the sentences containing antecedent 1 We use a b a \u03c6 to denote a zero anaphor, where the subscript a is the index of the zero anaphor itself and the superscript b is the index of the referent. A single without any script represents an intrasentential zero anaphor. Also note that a superscript attached to an NP is used to represent the index of the referent. and anaphor, it can further be divided into two types: immediate and long distance.", "cite_spans": [ { "start": 161, "end": 162, "text": "1", "ref_id": null } ], "ref_spans": [], "eq_spans": [], "section": "Zero Anaphora in Chinese", "sec_num": "2." }, { "text": "The former is where the sentence containing the antecedent is immediately followed by the one containing the anaphor, such as j Since Chinese has no inflection, conjugation, or case markers, the pronominal system is relatively simple, as shown in Table 1 [LT81] . A third-person pronoun can be used to replace an intersentential zero anaphor, except for first-and second-person pronouns, without changing the meaning of the sentence. Though the resulting meaning of each sentence is unchanged, the whole discourse becomes less coherent. ", "cite_spans": [ { "start": 255, "end": 261, "text": "[LT81]", "ref_id": "BIBREF14" } ], "ref_spans": [ { "start": 247, "end": 254, "text": "Table 1", "ref_id": "TABREF1" } ], "eq_spans": [], "section": "Zero Anaphora in Chinese", "sec_num": "2." }, { "text": "Centering has its computational foundations established by Grosz and Sidner [Gro77, Sid79] and were further developed by Groze, Joshi and Weinstein [GJW83, GJW95] .", "cite_spans": [ { "start": 76, "end": 83, "text": "[Gro77,", "ref_id": "BIBREF7" }, { "start": 84, "end": 90, "text": "Sid79]", "ref_id": "BIBREF18" }, { "start": 148, "end": 155, "text": "[GJW83,", "ref_id": "BIBREF2" }, { "start": 156, "end": 162, "text": "GJW95]", "ref_id": "BIBREF4" } ], "ref_spans": [], "eq_spans": [], "section": "Centering Model", "sec_num": "3." }, { "text": "Within the framework of the centering model, each utterance U in a discourse segment has two structures associated with it, called forward-looking centers, C f (U), and backward-looking center, C b (U). The forward-looking centers of U n , C f (U n ), depend only on the expressions that constitute that utterance. They are not constrained by features of any previous utterance in the discourse segment (DS), and the elements of C f (U n ) are partially ordered to reflect relative prominence in U n . The more highly ranked an element of C f (U n ), the more likely it is to be C b (U n+1 ). The highest ranked", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Centering Model", "sec_num": "3." }, { "text": "element of C f (U n ) that is realized 2 in U n+1 is the C b (U n+1 ).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Centering Model", "sec_num": "3." }, { "text": "The set of forward-looking centers, C f , is ranked according to discourse salience.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Centering Model", "sec_num": "3." }, { "text": "The highest ranked member of the set of forward-looking centers is referred to as the preferred center, C p . 3 The preferred center of the utterance U n represents a prediction about the C b of the following utterance U n+1 and is the most preferred antecedent of an anaphoric or elliptical expression in U n+1 . Hence, the most important single construct of the centering model is the ordering of the list of forward-looking centers [WIC94, SH96] .", "cite_spans": [ { "start": 110, "end": 111, "text": "3", "ref_id": null }, { "start": 435, "end": 442, "text": "[WIC94,", "ref_id": "BIBREF21" }, { "start": 443, "end": 448, "text": "SH96]", "ref_id": "BIBREF17" } ], "ref_spans": [], "eq_spans": [], "section": "Centering Model", "sec_num": "3." }, { "text": "In addition to the structures for centers, C b , and C f , the theory of centering specifies a set of constraints and rules [WIC94, GJW95] .", "cite_spans": [ { "start": 124, "end": 131, "text": "[WIC94,", "ref_id": "BIBREF21" }, { "start": 132, "end": 138, "text": "GJW95]", "ref_id": "BIBREF4" } ], "ref_spans": [], "eq_spans": [], "section": "Constraints and rules", "sec_num": "3.1" }, { "text": "For each utterance U i in a discourse segment U 1 , \u2026, U m :", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Constraints", "sec_num": null }, { "text": "1. U i has exactly one C b . 2. Every element of C f (U i ) must be realized in U i . 3. Ranking of elements in C f (U i ) guides determination of C b (U i+1 ).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Constraints", "sec_num": null }, { "text": "C b (U i ) is from C f (U i-1 )", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "The choice of", "sec_num": "4." }, { "text": ", and can not be from C f (U i-2 ) or other prior sets of C f .", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "The choice of", "sec_num": "4." }, { "text": "Backward-looking centers, C b s, are often omitted or pronominalized and discourses that continue centering the same entity are more coherent than those that shift from one center to another. This means that some transitions are preferred over others.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "The choice of", "sec_num": "4." }, { "text": "These observations are encapsulated in two rules [WIC90, WIC94, GJW95]:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "The choice of", "sec_num": "4." }, { "text": "For each utterance U i in a discourse segment U 1 , \u2026 , U m : Rule II reflect the intuition that continuation of the center and the use of retentions when possible to produce smooth trans itions to a new center provide a basis for local coherence. The transition states are further described in the next section.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rules", "sec_num": null }, { "text": "I. I. If any element of C f (U i ) is", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rules", "sec_num": null }, { "text": "The typology of transitions from U i-1 to U i is based on two factors: whether the C b (U i )", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "is the same as C b (U i-1 ), and whether this discourse entity, C b (U i ), is the same as the ", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "C p (U i ): 1. C b (U i ) = C b (U i-1 ), or C b (U i-1 ) is undefined. 2. C b (U i ) = C p (U i ) If", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": ") = C b (U i-1 ), or C b (U i-1 )", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "is undefined\" and \" C b", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "(U i ) = C p (U i ), or C b (U i ) is undefined.\" In (1c), the transition state is RETAIN because of \"C b (U 1c ) C p (U 1c )", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": ".\". SMOOTH-SHIFT is the last transition state of example (1) while \"", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "C b (U 1d ) = C p (U 1d )\" and \" C b (U 1d ) C b (U 1c )\"", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "hold. ", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "C b : undefined C f : [ , ] (1a) C p : CONTINUE C b : C f : [ZA ( )] (1b) C p ZA ( ) CONTINUE C b : C f : [ ] (1c) C p : RETAIN C b : C f : [ZA ( ), ] (1d) C p : ZA ( ) SMOOTH-SHIFT", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Transition states", "sec_num": "3.2" }, { "text": "This paper is concerned with resolving the problem of zero anaphora in Chinese using the centering model. In this section, we first describe the methodology of zero anaphora resolution we adopted based on centering. Second, we explain how to apply our rules and represent the results of applying the different rules to the test texts.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment and Result", "sec_num": "4." }, { "text": "C b (U i ) = C b (U i-1 ) or C b (U i-1 ) is undefined C b (U i ) C b (U i-1 ) C b (U i ) = C p (U i ) CONTINUE SMOOTH-SHIFT C b (U i ) C p (U i ) RETAIN ROUGH-SHIFT", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment for zero anaphora resolution", "sec_num": "4.1" }, { "text": "The task of zero and nominal anaphora resolution is performed after the semantic interpretation phase that converts the syntactic structure of a sentence into a semantic representation form such as the logic form [JA94] . After semantic interpretation, an anaphor becomes a parameter in a logic form. For example, the logic form of the (5b) is ( ). The task of anaphora resolution is to find out the referent of the omission in the logic forms. as the object of a prepositional phrase [LT81] . Therefore to apply the centering model for zero anaphora resolution, the essential task is to rank the elements in the set. The task of ranking elements is determined according to certain rules, for example Rule 2 described previously in Section 1. In this paper, our goal is to develop effective rules to obtain better result.", "cite_spans": [ { "start": 213, "end": 219, "text": "[JA94]", "ref_id": "BIBREF10" }, { "start": 485, "end": 491, "text": "[LT81]", "ref_id": "BIBREF14" } ], "ref_spans": [], "eq_spans": [], "section": "Experiment for zero anaphora resolution", "sec_num": "4.1" }, { "text": "We performed an experiment to examine the effectiveness of using a rule for the resolution of zero anaphors. In the experiment, we selected a number of industry news as the test text s. Table 4 summarizes the total news, paragraphs, utterances, zero anaphors and words in the test texts. In the experiment, we first of all identify by hand the referent of each zero anaphor occurring in the texts. Then we compute the referents of zero anaphors identified by using an algorithm employing a resolution rule. The computed result is then compared with the one by hand to see the correction rate of the resolution rule.", "cite_spans": [], "ref_spans": [ { "start": 186, "end": 193, "text": "Table 4", "ref_id": "TABREF5" } ], "eq_spans": [], "section": "Experiment for zero anaphora resolution", "sec_num": "4.1" }, { "text": "The correction rate of a resolution rule is defined as below.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment for zero anaphora resolution", "sec_num": "4.1" }, { "text": "Correction rate: Assume that m ZAs occur in n utterances. The correction rate of a resolution rule is the number of referents of ZAs resolved by an algorithm employing the resolution rule that are identical to the ones identified by hand.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment for zero anaphora resolution", "sec_num": "4.1" }, { "text": "The experiment is performed repeatedly by replacing new rules and it is stopped until promising result is obtained. The initial result of using Rule 2 shows that only 55% of the ZAs are correctly resolved, which is obviously not effective enough. The errors occurs in the initial result may be that Rule 2 does contain enough semantic knowledge. In the following, we propose other rules to replace Rule 2 and compare the results.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment for zero anaphora resolution", "sec_num": "4.1" }, { "text": "Grosz et al., in their paper [GJW95] , assume that grammatical roles are the major determinant for ranking the forward-looking centers, with the order \"Subject > Object(s) > Others\". In Chinese, the concept of subject seems to be less significant while the topic in a sentence appears to be crucial in explaining the structure of ordinary sentences in the language [LT81] . ) is realized by both specific nouns in the lexicon and a ZA having the highest priority of grammatical role criteria in U i+1 then the C b (U i+1 ) must be realized by C b (U i ).", "cite_spans": [ { "start": 29, "end": 36, "text": "[GJW95]", "ref_id": "BIBREF4" }, { "start": 365, "end": 371, "text": "[LT81]", "ref_id": "BIBREF14" } ], "ref_spans": [ { "start": 374, "end": 375, "text": ")", "ref_id": null } ], "eq_spans": [], "section": "Results of using other rules", "sec_num": "4.2" }, { "text": "In Rule 4, in addition to grammatical role criteria, we further add the lexical semantic knowledge to the nouns specified in the lexicon. The experiment results of using these rules are investigated as follows.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Results of using other rules", "sec_num": "4.2" }, { "text": "The experiment is performed three times by using Rule2, 3 and 4, respectively. The first experiment employs the simplest rule, Rule 2, as described in Section 1. Since", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment results using three rules", "sec_num": "4.3" }, { "text": "Rule 2 does not have constraint to order elements in C f , here we take the surface order of entities from left to right in the utterance. After performing the experiment, the correction rate is 55%, which is obviously not satisfied. In the second experiment, we employed an enhanced rule, Rule 3, and the correction rate is 62%. The result is better but it is still not significant. In the third experiment, we used a further enhanced rule, Rule 4, the correction rate becomes 85%, which is more promising. The results are summarized in Table 5 . having their antecedents outside this scope, the rules would be ineffective. Worse yet, the miss-resolution of a long distance zero anaphora would fail to resolve the following zero anaphors, or error chaining [SH96] . To solve this problem, we extend the referent set of C b (U i ) to be the collection of entities occurring in utterances previously in the discourse, that is, U 1 \u2026 U i-1 . The referent of a long distance zero anaphor is then determined by examining the elements in the extended referent set.", "cite_spans": [ { "start": 758, "end": 764, "text": "[SH96]", "ref_id": "BIBREF17" } ], "ref_spans": [ { "start": 538, "end": 545, "text": "Table 5", "ref_id": "TABREF7" } ], "eq_spans": [], "section": "Experiment results using three rules", "sec_num": "4.3" }, { "text": "The algorithm for resolving long distance zero anaphors is described as below.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment results using three rules", "sec_num": "4.3" }, { "text": "Description: A long distance zero anaphor z is found in the current utterance U i and then it enters the following procedure. Assume that the extended referent set is E. A temporary set, temp_set, is used to record the elements in E that satisfy the semantic constraints of z.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Experiment results using three rules", "sec_num": "4.3" }, { "text": "For each element e in E do If e satisfies the semantic constraints of z, then add e to temp_set. end for; If there is one element in temp_set then return the element as the result; else return the element in temp_set having longest distance from z as the result.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Procedure:", "sec_num": null }, { "text": "The s emantic constraints we used in the above procedure come from the selectional restrictions of the main verb in utterance U i [JA94] . This kind of restrictions can be used to select the referents of zero anaphors in the topic position.", "cite_spans": [ { "start": 130, "end": 136, "text": "[JA94]", "ref_id": "BIBREF10" } ], "ref_spans": [], "eq_spans": [], "section": "Procedure:", "sec_num": null }, { "text": "On the one hand, in the sentence which the topic and subject are identical, the zero anaphor in the topic position is restricted by the semantics of the main verb. On the other hand, for sentences with both topic and subject, the topic is frequently moved from the object positio n of the sentence. Thus zero anaphors of this sort are restricted by the main verb as well. We ignore the selectional restrictions of other syntactic constructs such as coverb and adjective phrases because the objects or heads of these kinds of phrases can not be zeroed according to syntactic constraints in Chinese [LT81] . Consider, for example, the long distance zero anaphor i 2 \u03c6 in (6d). Before entering the above procedure, assume that the extended referent set, {", "cite_spans": [ { "start": 597, "end": 603, "text": "[LT81]", "ref_id": "BIBREF14" } ], "ref_spans": [], "eq_spans": [], "section": "Procedure:", "sec_num": null }, { "text": "i , j ,", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Procedure:", "sec_num": null }, { "text": "k , l }, was obtained, where the first two elements satisfy the selectional restrictions of the main verb of (6d), . Here the first one is selected because it is in a more prominent position. c.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Procedure:", "sec_num": null }, { "text": "The NTD's exchange rate stops to slowly fall down.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "l k", "sec_num": null }, { "text": "d. i 2 \u03c6 j j", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "l k", "sec_num": null }, { "text": "(They) expect that Central Bank of China will not let NTD be depreciated.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "l k", "sec_num": null }, { "text": "The goal of this paper is to resolve zero anaphors occurring in discourses based on the centering model. A discourse is a sequence of utterances exhibiting coherence [GJW95] . The resolution of zero anaphors in a discourse is therefore divided into two parts. First, we process each utterance in turn and identify zero anaphors occurring in the utterance. Then we apply a zero anaphor resolution algorithm to resolve the referents of the zero anaphors.", "cite_spans": [ { "start": 166, "end": 173, "text": "[GJW95]", "ref_id": "BIBREF4" } ], "ref_spans": [], "eq_spans": [], "section": "Implementation", "sec_num": "6." }, { "text": "The first part consists of tasks of word segmentation, parsing and semantic", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Implementation", "sec_num": "6." }, { "text": "interpretation. An input utterance is fragmented into word sequence, and after parsing and semantic interpretation, the semantic form is obtained. Therefore, in this part, the input is a sequence of utterances and the output is the corresponding sequence of semantic forms. Zero anaphors with the information of either immediate or long distance are represented as arguments in the semantic forms. Basically, a zero anaphor is considered an immediate one. But if there are linguistic cues accompanied with the utterance, such as the utterance is the beginning of a new full sentence, and it has initial adverbial connectives, etc., then the zero anaphor is considered a long distance case. In the second part, the resolution procedure examines each zero anaphor in turn.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Implementation", "sec_num": "6." }, { "text": "If an immediate zero anaphor is found, then apply the resolution rules described in Section 4. Otherwise, if it is a long distance zero anaphor, then apply the procedure as described in Section 5. The system architecture is shown in Figure 2 . is build as a sentence-level parser in DCG [GM89] . Each utterance within an input discourse segme nt is converted into a syntactical structure by the parser and the output structure is interpreted to produce the semantic form, which includes the entities in the utterance and is also used to judge whether the utterance contains zero anaphors or not.", "cite_spans": [ { "start": 287, "end": 293, "text": "[GM89]", "ref_id": "BIBREF5" } ], "ref_spans": [ { "start": 233, "end": 241, "text": "Figure 2", "ref_id": "FIGREF7" } ], "eq_spans": [], "section": "Implementation", "sec_num": "6." }, { "text": "ZA resolution by consulting the domain ontology and resolution rules is the second part of our system. If an input utterance contains a zero anaphor, then apply the resolution rules described in Section 4 to obtain the referent of the zero anaphor.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Implementation", "sec_num": "6." }, { "text": "Currently, the ZA Resolution only deals with the immediate zero anaphors. We will extend the algorithm to include the resolution of long distance zero anaphors described in Section 5.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Implementation", "sec_num": "6." }, { "text": "In this paper, we performed the experiments on zero anaphora resolutio n in Chinese based on centering model. In the experiments, 85% of zero anaphors in the test texts were correctly resolved. The remaining zero anaphors were miss-resolved because of lack of sufficient domain knowledge and occurrence of long distance zero anaphors.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Conclusions", "sec_num": "7." }, { "text": "Since the centering model only focuses on local coherence in discourse, we therefore propose to extend the referent set of a zero anaphor to include all entities occurring previously in the discourse. Though the experiment results are promising to some extent, we found that there are problems that are worth further study. First we need to build domain ontology to get better resolution. Second, the phenomenon of error chaining is inherent in zero anaphors resolution. Thus an effective method is needed to account for this problem. The method we proposed in Section 5 is a step towards solving this problem. Third, the test texts used in this paper were selected from industry news. We will further extend our experiment to include texts from other domains.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Conclusions", "sec_num": "7." }, { "text": "An utterance U, realizes c if c is an element of the situation described by U, or c is the semantics interpretation of come subpart of U.3 The notion of preferred center corresponds to Sider's notion of expected focus[Sid83]", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null } ], "back_matter": [], "bib_entries": { "BIBREF0": { "ref_id": "b0", "title": "Hanyu lingxin huizhi de huayu fenxi (a discourse approach to zero anaphora in chinese) (in chinese)", "authors": [ { "first": "P", "middle": [], "last": "Chen", "suffix": "" } ], "year": 1987, "venue": "Zhongguo Yuwen (Chinese Linguistics)", "volume": "", "issue": "", "pages": "363--378", "other_ids": {}, "num": null, "urls": [], "raw_text": "P. Chen. 1987. 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Of COLING-90, Appendix, 6pp.", "links": null }, "BIBREF21": { "ref_id": "b21", "title": "Japan Discourse and the Process of Centering", "authors": [ { "first": "M", "middle": [ "A" ], "last": "Walker", "suffix": "" }, { "first": "M", "middle": [], "last": "Iida", "suffix": "" }, { "first": "S", "middle": [], "last": "Cote", "suffix": "" } ], "year": 1994, "venue": "Computational Linguistics", "volume": "20", "issue": "2", "pages": "193--233", "other_ids": {}, "num": null, "urls": [], "raw_text": "Walker, M. A., M. Iida and S. Cote, 1994. Japan Discourse and the Process of Centering. Computational Linguistics, 20(2): 193-233.", "links": null }, "BIBREF22": { "ref_id": "b22", "title": "Generation of Anaphors in Chinese", "authors": [ { "first": "Ching-Long", "middle": [], "last": "Yeh", "suffix": "" } ], "year": 1995, "venue": "", "volume": "", "issue": "", "pages": "", "other_ids": {}, "num": null, "urls": [], "raw_text": "Ching-Long Yeh. 1995. Generation of Anaphors in Chinese, Ph.D. dissertation, University of Edinburgh", "links": null } }, "ref_entries": { "FIGREF0": { "num": null, "uris": null, "text": "Electronics stocks were affected by high-tech stocks fallen heavily in America.b. i(Electronics stocks) continued falling down today.c.Securities stocks also had respondence.d. j (Securities stocks) fell by close one after another on the market.", "type_str": "figure" }, "FIGREF1": { "num": null, "uris": null, "text": "in (4b) and k 1 \u03c6 in (4d). For the long distance type, the sentence containing the antecedent and anaphors, on the other hand, are not in immediately succeeding order, such as i must use the tips of feet on one side to grasp the ground. then uses the feet on the other side to move upwards. pushes (its) body towards one side.", "type_str": "figure" }, "FIGREF2": { "num": null, "uris": null, "text": "realized by a pronoun in U i+1 then the C b (U i+1 ) must be realized by a pronoun also. II. Sequences of continuation are preferred over sequence of retaining; and sequences of retaining are to be preferred over sequences of shifting. Rule I represents one function of pronominal reference: the use of a pronoun to realize the C b signals the hearer that the speaker is continuing to talk about the same thing. Psychological research and cross-linguistic research have validated that the C b is preferentially realized by a pronoun in English and by equivalent forms (i.e. zero anaphora) in other languages [GJW95].", "type_str": "figure" }, "FIGREF3": { "num": null, "uris": null, "text": "Zhangsan bought an apple.b. i (It) is very fresh. Recall that the centering model, an utterance, U i , is associated with a set of forward-looking centers, C f , with each element an entity in U i . The highest ranked element in the set, C p , becomes the prediction of backward-looking centers, C b , of the following utterance, which is zeroed if it does not violate syntactic constraints, such", "type_str": "figure" }, "FIGREF4": { "num": null, "uris": null, "text": "By adopting the concept of grammatical roles and topic-prominence in Chinese, we order the grammatical roles in Chinese with topic having the highest priority as shown in Figure 1. The subject and objects occurring in an embedded clause, that is, Secondary Subject and Secondary Objects, are give lower priority.", "type_str": "figure" }, "FIGREF5": { "num": null, "uris": null, "text": "Grammatical role criteriaBy adding the grammatical role criteria to Rule 2, we obtain a new rule, Rule 3:", "type_str": "figure" }, "FIGREF6": { "num": null, "uris": null, "text": "People on the market worry that Central Bank will intervene the exchange rate again. b. i 1 \u03c6 (They) are afraid to enter the exchange market.", "type_str": "figure" }, "FIGREF7": { "num": null, "uris": null, "text": "System architecture In the system the N LP M odule carries out in order the work of word segmentation, parsing and semantic interpretation by consulting the lexicon and the syntactic and semantic rules. This module corresponds to the first part described previously in this section. According to the segmentation standard proposed by Academia Sinica [HCC96, HC+97], we built a small lexicon for the test texts, and employ a simple algorithm of word segmentation. The algorithm is according to a strategy that prioritizes the longest word first. The syntactic grammar rules we construct are from the utterances of the test texts and refer to Sinica Corpus and Auto tag program [SC01, CKIP99] and then the parser corresponding to the grammar rules", "type_str": "figure" }, "TABREF1": { "html": null, "content": "
NumberPersonPronoun
singularfirst
singularsecond,
singularthird, ,
pluralfirst
pluralsecond,
pluralthird,,
", "text": "Pronominal system in Chinese", "num": null, "type_str": "table" }, "TABREF2": { "html": null, "content": "", "text": "both (1) and (2) hold then a pair continuations across U n and across U n+1 . If (1) holds but (2) does not then the utterances are in a retaining transition, which corresponds to a situation where the speaker is intending to shift onto a new entity in the next utterance. If (1) does not hold then the utterances are in one of the shifting transition states depending on whether or not (2) holds. The definition of transition", "num": null, "type_str": "table" }, "TABREF3": { "html": null, "content": "
", "text": "Transition statesFor illustration purpose, consider the example (1) in Section 1; in theTable 3, the centering structures contain C b , C f and C p where the set of C f are partially ordered to reflect relative prominence in each utterance. The first two transition states of (1a) and", "num": null, "type_str": "table" }, "TABREF4": { "html": null, "content": "
", "text": "", "num": null, "type_str": "table" }, "TABREF5": { "html": null, "content": "
ParagraphsUtterancesWordsZero Anaphors
143619925
23262299
343121313
442921315
532720811
643528215
732823414
832728912
Total282391867115
", "text": "Summary of test texts", "num": null, "type_str": "table" }, "TABREF6": { "html": null, "content": "
Rule 4:
", "text": "For each utterance U i in a discourse segment U 1 , \u2026 , U m : If C b (U i ) is realized by a ZA in U i+1 and no other noun phrase having higher priority of grammatical role criteria than the ZA then the C Secondary Objects Rule 3 is used to verify if the order of the elements in grammatical role criteria we assumed is helpful to raise the correction rate of zero anaphora resolution. We further developed another rule, Rule 4 , by considering the domain knowledge corresponding to the test texts. For each utterance U i in a discourse segment U 1 , \u2026 , U", "num": null, "type_str": "table" }, "TABREF7": { "html": null, "content": "
Rule 2Rule 3Rule 4
ZAs correctly resolved637198
Correction Rate55%62%85%
5. Discussions
", "text": "Summary of experiment results using three rulesWe have performed experiments on ZA resolution by using three rules with different complexities. The result is promising to some extent; however, there are still 15% of ZAs in the test texts can not be correctly resolved. In the following, we investigate the problems and propose methods to deal with them. One problem is because of insufficient semantic knowledge, namely domain ontology. In the lexical database, one word may have several word senses and there is a set of synonyms for each sense[MBF+90]. Besides, one word may have hypernyms, hyponyms, coordinate sisters, and other relationship to another word, e.g.,", "num": null, "type_str": "table" } } } }