emnlp emnlp2013 emnlp2013-45 knowledge-graph by maker-knowledge-mining

45 emnlp-2013-Chinese Zero Pronoun Resolution: Some Recent Advances

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Author: Chen Chen ; Vincent Ng

Abstract: We extend Zhao and Ng's (2007) Chinese anaphoric zero pronoun resolver by (1) using a richer set of features and (2) exploiting the coreference links between zero pronouns during resolution. Results on OntoNotes show that our approach significantly outperforms two state-of-the-art anaphoric zero pronoun resolvers. To our knowledge, this is the first work to report results obtained by an end-toend Chinese zero pronoun resolver.

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sentIndex sentText sentNum sentScore

1 edu Abstract We extend Zhao and Ng's (2007) Chinese anaphoric zero pronoun resolver by (1) using a richer set of features and (2) exploiting the coreference links between zero pronouns during resolution. [sent-3, score-0.553]

2 Results on OntoNotes show that our approach significantly outperforms two state-of-the-art anaphoric zero pronoun resolvers. [sent-4, score-0.248]

3 To our knowledge, this is the first work to report results obtained by an end-toend Chinese zero pronoun resolver. [sent-5, score-0.179]

4 1 Introduction A zero pronoun (ZP) is a gap in a sentence that is found when a phonetically null form is used to refer to a real-world entity. [sent-6, score-0.224]

5 An anaphoric zero pronoun (AZP) is a ZP that corefers with one or more preceding noun phrases (NPs) in the associated text. [sent-7, score-0.277]

6 Unlike overt pronouns, ZPs lack grammatical attributes that are useful for overt pronoun resolution such as number and gender. [sent-8, score-0.631]

7 This makes ZP resolution more challenging than overt pronoun resolution. [sent-9, score-0.417]

8 We aim to improve the state of the art in Chinese AZP resolution by proposing two extensions. [sent-10, score-0.099]

9 First, while previous approaches to this taskhave primarily focused on employing positional and syntactic features (e. [sent-11, score-0.025]

10 , Zhao and Ng (2007) [Z&N;], Kong and Zhou (2010) [K&Z;]), we exploit a richer set of features for capturing the context of an AZP and its candidate antecedents. [sent-13, score-0.063]

11 Second, to alleviate the difficulty of resolving an AZP to an antecedent far away from it, we break down the process into smaller, intermediate steps, where we allow coreference links between AZPs to be established. [sent-14, score-0.334]

12 We apply our two extensions to a state-of-the-art Chinese AZP resolver proposed by Z&N; and eval1360 uate the resulting resolver on the OntoNotes corpus. [sent-15, score-0.299]

13 Experimental results show that this resolver significantly outperforms both Z&N;'s resolver and another state-of-the-art resolver proposed by K&Z. [sent-16, score-0.414]

14 ; It is worth noting that while previous work on Chinese ZP resolution has reported results obtained via gold information (e. [sent-17, score-0.125]

15 , using gold AZPs and extracting candidate antecedents and other features from gold syntactic parse trees), this is the first work to report the results of an end-to-end Chinese ZP resolver. [sent-19, score-0.256]

16 2 Baseline AZP Resolution Systems An AZP resolution algorithm takes as input a set of AZPs produced by an AZP identification system. [sent-24, score-0.126]

17 1 Anaphoric Zero Pronoun Identification We employ two steps to identify AZPs. [sent-27, score-0.034]

18 In the extraction step, we heuristically extract candidate ZPs. [sent-28, score-0.063]

19 To implement the extraction step, we use Z&N;'s and K&Z;'s observation: ZPs can only occur before a VP node in a syntactic parse tree. [sent-30, score-0.086]

20 However, accord- ing to K&Z;, ZPs do not need to be extracted from every VP: if a VP node occurs in a coordinate structure or is modified by an adverbial node, then only its parent VP node needs to be considered. [sent-31, score-0.052]

21 oc d2s0 i1n3 N Aastusorcaila Ltiaon g fuoarg Ceo Pmrpoucetastsi on ga,l p Laignegsu 1is3t6ic0s–1365, right of z (if iis positive) or the ith word to the left of z (if iis negative). [sent-35, score-0.04]

22 To implement the classification step, we train a classifier using SVMlight (Joachims, 1999) to distinguish AZPs from non-AZPs. [sent-40, score-0.018]

23 We employ 32 features, 13 of which were proposed by Z&N; and 19 of which were proposed by Yang and Xue (2010). [sent-41, score-0.034]

24 In our implementation of the Z&N; baseline, we use SVMlight to train a mention-pairmodel for determining whether an AZP z and a candidate antecedent of z are coreferent. [sent-45, score-0.206]

25 We consider all NPs preceding z that do not have the same head as its parent NP in the parse tree to be z's candidate antecedents. [sent-46, score-0.199]

26 's (2001) method to create training instances: we create a positive instance between an AZP, z, and its closest overt antecedent, and we create a negative instance between z and each of the 1361 intervening candidates. [sent-48, score-0.387]

27 Each instance is represented by the 26 features employed by Z&N. [sent-49, score-0.021]

28 During testing, we adopt the closest-first resolution strategy, resolving an AZP to the closest candidate antecedent that is classified as coreferent with it. [sent-51, score-0.455]

29 K&Z; employ a tree kernel-based approach to AZP resolution. [sent-53, score-0.058]

30 Like Z&N;, K&Z; (1) train a mentionpair model for determining whether an AZP z and a candidate antecedent of z are coreferent, (2) use Soon et al. [sent-54, score-0.206]

31 's method to create training instances, and (3) resolve an AZP to its closest coreferent candidate antecedent. [sent-55, score-0.265]

32 Unlike Z&N;, however, K&Z; usethe SVMlight−TK learning algorithm(Moschitti, 1When resolving a gold AZP z, if none of the preceding candidate antecedents is classified as coreferent with it, we resolve it to the candidate that has the highest coreference likelihood with it. [sent-56, score-0.503]

33 Here, we employ the signed distance from the SVM hyperplane to measure the coreference likelihood. [sent-57, score-0.088]

34 2006) to train their model, employing a parse subtree known as a dynamic expansion tree (Zhou et al. [sent-58, score-0.091]

35 AZPs are omitted subjects that precede VP nodes in a sentence's parse tree. [sent-62, score-0.065]

36 From the VP node, we can extract its head verb (Predz) and the head of its object NP (Obj), if any. [sent-63, score-0.046]

37 Note that Predz and Obj contain important contextual information for an AZP. [sent-64, score-0.035]

38 Next, observe that if a NP is coreferent with an AZP, it should be able to fill the AZP's gap and be compatible with the gap's context. [sent-65, score-0.123]

39 That means ∗pro∗ hope yth aarte our ivnigsit tohrast can try i. [sent-90, score-0.025]

40 If we try using them to fill this AZP's gap, we know based on selectional preferences that 他们 ? [sent-98, score-0.052]

41 We supply the AZP resolver with the following information to help it make these decisions. [sent-103, score-0.132]

42 First, we find the head word of each candidate antecedent, Headc. [sent-104, score-0.086]

43 Finally, we employ them as binary lexical features, setting their feature values to 1 if and only if they can be extracted from the instance under consideration. [sent-106, score-0.055]

44 A narrative event chain is a partially ordered set ofevents related by a common protagonist (Chambers and Jurafsky, 2008). [sent-109, score-0.155]

45 For example, we can infer from the chain "borrow-s invests spend-s lend-s" that a person who borrows (pre2We tried to apply Kehler et al. [sent-110, score-0.022]

46 's (2005) methods to learn Chinese selectional preferences from unlabeled data, but without success. [sent-112, score-0.035]

47 (The country gives our department money, but all ∗pro∗ provides is exactly what we worked for. [sent-136, score-0.02]

48 ) In E2, ∗pro∗ is coreferent with 国家 (The country), a En2d, ,t ∗hep presence oeffe threen tn a wrriathtiv 国e 家even (Tt hchea cionu ? [sent-137, score-0.076]

49 However, given the unavailability of induced or hand-crafted narrative chains in Chinese4, we make the simplifying assumption that two verbs form a lexical chain if they are lexically identical. [sent-144, score-0.17]

50 5 We create two features to exploit narrative event chains for a candidate NP, c, if it serves as a subject or object. [sent-145, score-0.259]

51 The first feature, which encodes whether narrative chains are present, has three possible values: 0 if Predc and Predz are not the same; 1 if Predc and Predz are the same and c is a subject; and 2 if Predc and Predz are the same and c is an object. [sent-147, score-0.13]

52 The second feature is a binary lexical feature, Predc+Predz+Subject/Object; its value is 1 if and only if Predc, Predz, and Subject/Object can be found in the associated instance, where Subject/Object denotes the grammatical role of c. [sent-148, score-0.018]

53 We observe that the punctuation (Punc) at the end of a sentence where an AZP occurs also provides contextual information, especially in conversation documents. [sent-150, score-0.053]

54 In conversations, if a sentence containing an AZP ends with a 3"-s" denotes the fact that the protagonist serves as the grammatical subject in these events. [sent-151, score-0.065]

55 4We tried to construct narrative chains for Chinese using both learning-based and dictionary-based methods. [sent-152, score-0.13]

56 Specifically, we induced narrative chains using Chambers and Juraf- sky's (2008) method, but were not successful owing to the lack of an accurate Chinese coreference resolver. [sent-153, score-0.184]

57 In addition, we constructed narrative chains using both lexically identical verbs and the synonyms obtained from a WordNet-like Chinese resource called Tongyicicilin, but they did not help improve resolution performance. [sent-154, score-0.247]

58 5Experiments on the training data show that if an AZP and a candidate antecedent are subjects of (different occurrences of) the same verb, then the probability that the candidate antecedent is coreferent with the AZP is 0. [sent-155, score-0.511]

59 This result suggests that our assumption, though somewhat simplistic, is useful as far as AZP resolution is concerned. [sent-157, score-0.12]

60 e rTe,o capture ethfeisrs information, we cer sepaetea a binary lexical feature, Headc+Punc, whose value is 1 if and only if Headc and Punc appear in the instance under consideration. [sent-225, score-0.021]

61 1 Motivation Like an overt pronoun, a ZP whose closest overt antecedent is far away from it is harder to resolve than one that has a nearby overt antecedent. [sent-227, score-0.921]

62 2% of the AZPs appear in the same sentence as their closest overt antecedent, and 22. [sent-229, score-0.246]

63 7% of the AZPs appear two or more sentences away from their closest overt antecedent. [sent-230, score-0.28]

64 , AZPs whose closest overt antecedents are far away from them) are coreferential with nearby ZPs. [sent-233, score-0.434]

65 There are three AZPs (denoted by ∗proi∗, nwohmereen 1o ≤ ih ≤ 3), ahlrle eofA wZhPisch(d reenfoetre dtob tyh∗ep orvoe∗rt, pronoun 她≤ (She) )in, athlle o ffir wsth sicehnte renfceer. [sent-235, score-0.137]

66 oIn t hthei so example, it is fairly easy to resolve ∗pro1∗ correctly, ample, it is fa 6One may wonder whether we can similarly identify constraints on the antecedents of a ZP from clause conjunctions. [sent-236, score-0.15]

67 since its antecedent is the subject of previous sentence. [sent-239, score-0.162]

68 Together with the fact that there are many intervening candidate antecedents, it is not easy for a resolver to cor- rectly resolve ∗pro3∗. [sent-242, score-0.27]

69 To facilitate the resolution of∗pro3∗ and difficultto-Treosofalcviel AtatZePths einr general, we propose ntdhed iffoflilcouwlt-ing idea. [sent-243, score-0.099]

70 We allow an AZP resolver to (1) establish coreferent links between two consecutive ZPs (i. [sent-244, score-0.266]

71 , ∗pro1∗−∗pro2∗ and ∗pro2∗−∗pro3∗ in our example), w∗h−ic∗hp are presumably easy ptor oes∗tab inlis ohu rb e ecxaaumsethe two AZPs involved are close to each other; and then (2) treat them as bridges and infer that ∗pro3∗'s tohveenrt ( 2a)nt terecaetdte hnetm mis a她s b (She). [sent-246, score-0.048]

72 2 Modified Resolution Algorithm We implement the aforementioned idea by modifying the AZP resolver as follows. [sent-248, score-0.15]

73 When we resolve an AZP z during testing, we augment the set of candidate antecedents for z with the set ofAZPs preceding z. [sent-249, score-0.26]

74 Since we have only specified how to compute features for instances composed of an AZP and an overt candidate antecedent thus far (see Section 2. [sent-250, score-0.423]

75 Hence, by the time we resolve an AZP z, all the AZPs preceding z have been resolved. [sent-253, score-0.098]

76 Hence, when we create a test instance ibetween z and one of the preceding AZPs (say y), we create ias if the gap y was filled with the smallest tree embedding the NP to which y was resolved. [sent-254, score-0.172]

77 By allowing coreference links between (presumably nearby) ZPs to be established, we can reason over the resulting coreference links, treating them as bridges that can help us find an overt antecedent that is far away from an AZP. [sent-255, score-0.59]

78 For evaluation, we employ the portion of the OntoNotes 4. [sent-267, score-0.034]

79 We consider a ZP anaphoric if i ts i ms acrokreedfe aresn ∗tiparl ow∗. [sent-273, score-0.051]

80 We express the results of both AZP identification and AZP resolution in terms of recall (R), precision (P) and F-score (F). [sent-276, score-0.126]

81 2 Results and Discussion The three major columns of Table 4 show the results obtained in three settings, which differ in terms of whether gold/system AZPs and manually/automatically constructed parse trees are used to extract candidate antecedents and features. [sent-278, score-0.229]

82 In the first setting, the resolvers are provided with gold AZPs and goldparse trees. [sent-279, score-0.109]

83 As we can see, the Z&N; baseline significantly outperforms the K&Z; baseline by 3. [sent-281, score-0.018]

84 7 Adding the contextual features, the ZP links, and both extensions to Z&N; increase its F-score significantly by 4. [sent-283, score-0.088]

85 In the next two settings, the resolvers operate on the system AZPs provided by the AZP identification component. [sent-287, score-0.11]

86 When gold parse trees are employed, the recall, precision and F-score of AZP identification are 50. [sent-288, score-0.12]

87 Column 2 shows the results of the resolvers obtained 7All significance tests are paired t-tests, with p < 0. [sent-292, score-0.083]

88 As we can see, Z&N; again significantly outperforms K&Z; by 3. [sent-295, score-0.018]

89 Adding the contextual features, the ZP links, and both extensions to Z&N; increase its F-score by 3. [sent-297, score-0.07]

90 The system with contextual features and the full system both yield results that are significantly better than those of the Z&N; baseline. [sent-301, score-0.053]

91 A closer examination of the results reveals why the ZP links are not effective in improving performance: when employing system AZPs, many erroneous ZP links were introduced to the system. [sent-302, score-0.157]

92 Column 3 shows the results of the resolvers when we employ system AZPs and the automatically generated parse trees provided by the CoNLL-2012 shared task organizers to compute candidate antecedents and features. [sent-303, score-0.346]

93 Using automatic parse trees, the performance on AZP identification drops to 30. [sent-306, score-0.069]

94 Incorporating the contextual features, the ZP links, and both extensions increase the F-score by 1. [sent-311, score-0.07]

95 The system with contextual features and the full system both yield results that are significantly better than those of the Z&N; baseline. [sent-315, score-0.053]

96 6 Conclusions We proposed two extensions to a state-of-theart Chinese AZP resolver proposed by Zhao and Ng (2007). [sent-316, score-0.167]

97 Experimental results on the OntoNotes dataset showed that the resulting resolver significantly improved both Zhao and Ng's and Kong and Zhou's (2010) resolvers, regardless of whether gold or system AZPs and syntactic parse trees are used. [sent-317, score-0.243]

98 Context-sensitive convolution tree kernel for pronoun edoxp in rioe stn es ,dcfein sdtiahnrigsly p,racepofenl rclautrsetihotehnosvioe rworfsectoh remoafmueitchniodar lstpiao n lids- Jpreaosigonetlust2Ci5o n- . [sent-324, score-0.146]

99 A tree kernelbased unified framework for Chinese zero anaphora resolution. [sent-342, score-0.081]

100 A machine learning approach to coreference resolution of noun phrases. [sent-350, score-0.153]

similar papers computed by tfidf model

tfidf for this paper:

wordName wordTfidf (topN-words)

[('azp', 0.711), ('azps', 0.347), ('zp', 0.277), ('overt', 0.196), ('predz', 0.156), ('antecedent', 0.143), ('resolver', 0.132), ('pronoun', 0.122), ('zps', 0.106), ('resolution', 0.099), ('antecedents', 0.099), ('pro', 0.09), ('predc', 0.087), ('narrative', 0.083), ('resolvers', 0.083), ('coreferent', 0.076), ('chinese', 0.071), ('vp', 0.07), ('headc', 0.069), ('candidate', 0.063), ('links', 0.058), ('zero', 0.057), ('coreference', 0.054), ('punc', 0.052), ('anaphoric', 0.051), ('resolve', 0.051), ('closest', 0.05), ('chains', 0.047), ('preceding', 0.047), ('kong', 0.043), ('ontonotes', 0.042), ('parse', 0.042), ('zhao', 0.039), ('obj', 0.038), ('svmlight', 0.038), ('contextual', 0.035), ('extensions', 0.035), ('oin', 0.035), ('nearby', 0.034), ('employ', 0.034), ('away', 0.034), ('np', 0.03), ('gap', 0.03), ('bridges', 0.03), ('kehler', 0.03), ('zhou', 0.028), ('protagonist', 0.028), ('identification', 0.027), ('soon', 0.026), ('gold', 0.026), ('node', 0.026), ('ow', 0.026), ('chambers', 0.026), ('hope', 0.025), ('trees', 0.025), ('employing', 0.025), ('ng', 0.025), ('create', 0.025), ('resolving', 0.024), ('intervening', 0.024), ('tree', 0.024), ('head', 0.023), ('subjects', 0.023), ('event', 0.022), ('chain', 0.022), ('fang', 0.022), ('pronouns', 0.022), ('far', 0.021), ('instance', 0.021), ('iis', 0.02), ('living', 0.02), ('selectional', 0.02), ('money', 0.02), ('country', 0.02), ('subject', 0.019), ('guodong', 0.019), ('presumably', 0.018), ('lexically', 0.018), ('yang', 0.018), ('implement', 0.018), ('conversation', 0.018), ('significantly', 0.018), ('grammatical', 0.018), ('fill', 0.017), ('nps', 0.017), ('service', 0.017), ('guage', 0.016), ('hwee', 0.016), ('reveals', 0.016), ('tou', 0.016), ('column', 0.016), ('preferences', 0.015), ('speaker', 0.015), ('phonetically', 0.015), ('cne', 0.015), ('aleksandr', 0.015), ('chasing', 0.015), ('hep', 0.015), ('ih', 0.015), ('invest', 0.015)]

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Abstract: This paper addresses the task of predicting the correct French translations of third-person subject pronouns in English discourse, a problem that is relevant as a prerequisite for machine translation and that requires anaphora resolution. We present an approach based on neural networks that models anaphoric links as latent variables and show that its performance is competitive with that of a system with separate anaphora resolution while not requiring any coreference-annotated training data. This demonstrates that the information contained in parallel bitexts can successfully be used to acquire knowledge about pronominal anaphora in an unsupervised way. 1 Motivation When texts are translated from one language into another, the translation reconstructs the meaning or function of the source text with the means of the target language. Generally, this has the effect that the entities occurring in the translation and their mutual relations will display similar patterns as the entities in the source text. In particular, coreference patterns tend to be very similar in translations of a text, and this fact has been exploited with good results to project coreference annotations from one language into another by using word alignments (Postolache et al., 2006; Rahman and Ng, 2012). On the other hand, what is true in general need not be true for all types of linguistic elements. For instance, a substantial percentage ofthe English thirdperson subject pronouns he, she, it and they does not get realised as pronouns in French translations (Hardmeier, 2012). Moreover, it has been recognised 380 by various authors in the statistical machine translation (SMT) community (Le Nagard and Koehn, 2010; Hardmeier and Federico, 2010; Guillou, 2012) that pronoun translation is a difficult problem because, even when a pronoun does get translated as a pronoun, it may require choosing the correct word form based on agreement features that are not easily pre- dictable from the source text. The work presented in this paper investigates the problem of cross-lingual pronoun prediction for English-French. Given an English pronoun and its discourse context as well as a French translation of the same discourse and word alignments between the two languages, we attempt to predict the French word aligned to the English pronoun. As far as we know, this task has not been addressed in the literature before. In our opinion, it is interesting for several reasons. By studying pronoun prediction as a task in its own right, we hope to contribute towards a better understanding of pronoun translation with a longterm view to improving the performance of SMT systems. Moreover, we believe that this task can lead to interesting insights about anaphora resolution in a multi-lingual context. In particular, we show in this paper that the pronoun prediction task makes it possible to model the resolution of pronominal anaphora as a latent variable and opens up a way to solve a task relying on anaphora resolution without using any data annotated for anaphora. This is what we consider the main contribution of our present work. We start by modelling cross-lingual pronoun pre- diction as an independent machine learning task after doing anaphora resolution in the source language (English) using the BART software (Broscheit et al., 2010). We show that it is difficult to achieve satisfactory performance with standard maximumProceSe datintlges, o Wfa tsh ein 2g01to3n, C UoSnfAe,re 1n8c-e2 o1n O Ecmtopbier ic 2a0l1 M3.et ?hc o2d0s1 i3n A Nsastoucria lti Loan fgoura Cgoem Ppruotcaetsiosin agl, L piang eusis 3t8ic0s–391, The latest version released in March is equipped with ...It is sold at ... La dernière version lancée en mars est dotée de ... • est vendue ... • Figure 1: Task setup entropy classifiers especially for low-frequency pronouns such as the French feminine plural pronoun elles. We propose a neural network classifier that achieves better precision and recall and manages to make reasonable predictions for all pronoun categories in many cases. We then go on to extend our neural network architecture to include anaphoric links as latent variables. We demonstrate that our classifier, now with its own source language anaphora resolver, can be trained successfully with backpropagation. In this setup, we no longer use the machine learning component included in the external coreference resolution system (BART) to predict anaphoric links. Anaphora resolution is done by our neural network classifier and requires only some quantity of word-aligned parallel data for training, completely obviating the need for a coreference-annotated training set. 2 Task Setup The overall setup of the classification task we address in this paper is shown in Figure 1. We are given an English discourse containing a pronoun along with its French translation and word alignments between the two languages, which in our case were computed automatically using a standard SMT pipeline with GIZA++ (Och and Ney, 2003). We focus on the four English third-person subject pronouns he, she, it and they. The output of the classifier is a multinomial distribution over six classes: the four French subject pronouns il, elle, ils and elles, corresponding to masculine and feminine singular and plural, respectively; the impersonal pronoun ce/c’, which occurs in some very frequent constructions such as c’est (it is); and a sixth class OTHER, which indicates that none of these pronouns was used. In general, a pronoun may be aligned to multiple words; in this case, a training example is counted as a positive example for a class if the target word occurs among the words aligned to the pronoun, irrespective of the presence of other 381 word candidate training ex. verseiol ena0 0 1 01 10 0 0 .0510 .50 p 12= . 910. 5.9 050 Figure 2: Antecedent feature aggregation aligned tokens. This task setup resembles the problem that an SMT system would have to solve to make informed choices when translating pronouns, an aspect oftranslation neglected by most existing SMT systems. An important difference between the SMT setup and our own classifiers is that we use context from humanmade translations for prediction. This potentially makes the task both easier and more difficult; easier, because the context can be relied on to be correctly translated, and more difficult, because human translators frequently create less literal translations than an SMT system would. Integrating pronoun prediction into the translation process would require significant changes to the standard SMT decoding setup in order to take long-range dependencies in the target language into account, which is why we do not address this issue in our current work. In all the experiments presented in this paper, we used features from two different sources: Anaphora context features describe the source language pronoun and its immediate context consisting of three words to its left and three words to its right. They are encoded as vectors whose dimensionality is equal to the source vocabulary size with a single non-zero component indicating the word referred to (one-hot vectors). Antecedent features describe an antecedent candidate. Antecedent candidates are represented by the target language words aligned to the syntactic head of the source language markable TED News ce 16.3 % 6.4 % elle 7.1 % 10.1 % elles 3.0 % 3.9 % il 17.1 % 26.5 % ils 15.6 % 15.1 % OTHER 40.9 % 38.0 % – – Table 1: Distribution of classes in the training data noun phrase as identified by the Collins head finder (Collins, 1999). The different handling of anaphora context features and antecedent features is due to the fact that we always consider a constant number of context words on the source side, whereas the number of word vectors to be considered depends on the number of antecedent candidates and on the number of target words aligned to each antecedent. The encoding of the antecedent features is illustrated in Figure 2 for a training example with two antecedent candidates translated to elle and la version, respectively. The target words are represented as one-hot vectors with the dimensionality of the target language vocabulary. These vectors are then averaged to yield a single vector per antecedent candidate. Finally, the vectors of all candidates for a given training example are weighted by the probabilities assigned to them by the anaphora resolver (p1 and p2) and summed to yield a single vector per training example. 3 Data Sets and External Tools We run experiments with two different test sets. The TED data set consists of around 2.6 million tokens of lecture subtitles released in the WIT3 corpus (Cettolo et al., 2012). The WIT3 training data yields 71,052 examples, which were randomly partitioned into a training set of 63,228 examples and a test set of 7,824 examples. The official WIT3 development and test sets were not used in our experiments. The news-commentary data set is version 6 of the parallel news-commentary corpus released as a part of the WMT 2011training data1 . It contains around 2.8 million tokens ofnews text and yields 3 1,017 data points, 1http: //www. statmt .org/wmt11/translation-task. html (3 July 2013). 382 which were randomly split into 27,900 training examples and 3,117 test instances. The distribution of the classes in the two training sets is shown in Table 1. One thing to note is the dominance of the OTHER class, which pools together such different phenomena as translations with other pronouns not in our list (e. g., celui-ci) and translations with full noun phrases instead of pronouns. Splitting this group into more meaningful subcategories is not straightforward and must be left to future work. The feature setup of all our classifiers requires the detection of potential antecedents and the extraction of features pairing anaphoric pronouns with antecedent candidates. Some of our experiments also rely on an external anaphora resolution component. We use the open-source anaphora resolver BART to generate this information. BART (Broscheit et al., 2010) is an anaphora resolution toolkit consisting of a markable detection and feature extraction pipeline based on a variety of standard natural language processing (NLP) tools and a machine learning component to predict coreference links including both pronominal anaphora and noun-noun coreference. In our experiments, we always use BART’s markable detection and feature extraction machinery. Markable detection is based on the identification of noun phrases in constituency parses generated with the Stanford parser (Klein and Manning, 2003). The set of features extracted by BART is an extension of the widely used mention-pair anaphora resolution feature set by Soon et al. (2001) (see below, Section 6). In the experiments of the next two sections, we also use BART to predict anaphoric links for pronouns. The model used with BART is a maximum entropy ranker trained on the ACE02-npaper corpus (LDC2003T1 1). In order to obtain a probability distribution over antecedent candidates rather than onebest predictions or coreference sets, we modified the ranking component with which BART resolves pronouns to normalise and output the scores assigned by the ranker to all candidates instead of picking the highest-scoring candidate. 4 Baseline Classifiers In order to create a simple, but reasonable baseline for our task, we trained a maximum entropy (ME) ce TED (Accuracy: 0.685) P R 0.593 0.728 F 0.654 elle 0.798 0.523 elles 0.812 0.164 il 0.764 0.550 ils 0.632 0.949 OTHER 0.724 0.692 News commentary (Accuracy: 0.576) ce elle elles il ils OTHER P 0.508 0.530 0.538 0.600 0.593 0.564 R 0.294 0.312 0.062 0.666 0.769 0.609 Table 2: Maximum entropy classifier results 0.632 0.273 0.639 0.759 0.708 F 0.373 0.393 0.111 0.631 0.670 0.586 TED (Accuracy: 0.700) P R ce 0.634 0.747 elle 0.756 0.617 elles 0.679 0.319 il 0.719 0.591 ils 0.663 0.940 OTHER 0.743 0.678 News commentary (Accuracy: 0.576) F 0.686 0.679 0.434 0.649 0.778 0.709 P 0.477 0.498 F 0.400 0.444 ce elle R 0.344 0.401 elles il ils OTHER 0.565 0.655 0.570 0.567 0.116 0.626 0.834 0.573 0.193 0.640 0.677 0.570 Table 3: Neural network classifier with anaphoras resolved by BART classifier with the MegaM software package2 using the features described in the previous section and the anaphora links found by BART. Results are shown in Table 2. The baseline results show an overall higher accuracy for the TED data than for the newscommentary data. While the precision is above 50 % in all categories and considerably higher in some, recall varies widely. The pronoun elles is particularly interesting. This is the feminine plural of the personal pronoun, and it usually corresponds to the English pronoun they, which is not marked for gender. In French, elles is a marked choice which is only used if the antecedent exclusively refers to females or feminine-gendered objects. The presence of a single item with masculine grammatical gender in the antecedent will trigger the use of the masculine plural pronoun ils instead. This distinction cannot be predicted from the English source pronoun or its context; making correct predictions requires knowledge about the antecedent of the pronoun. Moreover, elles is a low-frequency pronoun. There are only 1,909 occurrences of this pro2http : //www . umiacs .umd .edu/~hal/megam/ (20 June 2013). 383 noun in the TED training data, and 1,077 in the newscommentary training set. Because of these special properties of the feminine plural class, we argue that the performance of a classifier on elles is a good indicator ofhow well it can represent relevant knowledge about pronominal anaphora as opposed to overfitting to source contexts or acting on prior assumptions about class frequencies. In accordance with the general linguistic preference for ils, the classifier tends to predict ils much more often than elles when encountering an English plural pronoun. This is reflected in the fact that elles has much lower recall than ils. Clearly, the classifier achieves a good part of its accuracy by making ma- jority choices without exploiting deeper knowledge about the antecedents of pronouns. An additional experiment with a subset of 27,900 training examples from the TED data confirms that the difference between TED and news commentaries is not just an effect of training data size, but that TED data is genuinely easier to predict than news commentaries. In the reduced data TED condition, the classifier achieves an accuracy of 0.673. Precision and recall of all classifiers are much closer to the Figure 3: Neural network for pronoun prediction large-data TED condition than to the news commentary experiments, except for elles, where we obtain an F-score of 0.072 (P 0.818, R 0.038), indicating that small training data size is a serious problem for this low-frequency class. 5 Neural Network Classifier In the previous section, we saw that a simple multiclass maximum entropy classifier, while making correct predictions for much of the data set, has a significant bias towards making majority class decisions, relying more on prior assumptions about the frequency distribution of the classes than on antecedent features when handling examples of less frequent classes. In order to create a system that can be trained to rely more explicitly on antecedent information, we created a neural network classifier for our task. The introduction of a hidden layer should enable the classifier to learn abstract concepts such as gender and number that are useful across multiple output categories, so that the performance of sparsely represented classes can benefit from the training examples of the more frequent classes. The overall structure of the network is shown in Figure 3. As inputs, the network takes the same features that were available to the baseline ME classifier, based on the source pronoun (P) with three words of context to its left (L1 to L3) and three words to its right (R1 to R3) as well as the words aligned to the syntactic head words of all possible antecedent candidates as found by BART (A). All words are 384 encoded as one-hot vectors whose dimensionality is equal to the vocabulary size. If multiple words are aligned to the syntactic head of an antecedent candidate, their word vectors are averaged with uniform weights. The resulting vectors for each antecedent are then averaged with weights defined by the posterior distribution of the anaphora resolver in BART (p1 to p3). The network has two hidden layers. The first layer (E) maps the input word vectors to a low-dimensional representation. In this layer, the embedding weights for all the source language vectors (the pronoun and its 6 context words) are tied, so if two words are the same, they are mapped to the same lowerdimensional embedding irrespective of their position relative to the pronoun. The embedding of the antecedent word vectors is independent, as these word vectors represent target language words. The entire embedding layer is then mapped to another hidden layer (H), which is in turn connected to a softmax output layer (S) with 6 outputs representing the classes ce, elle, elles, il, ils and OTHER. The non-linearity of both hidden layers is the logistic sigmoid function, f(x) = 1/(1 + e−x). In all experiments reported in this paper, the dimensionality of the source and target language word embeddings is 20, resulting in a total embedding layer size of 160, and the size of the last hidden layer is equal to 50. These sizes are fairly small. In experiments with larger layer sizes, we were able to obtain similar, but no better results. The neural network is trained with mini-batch stochastic gradient descent with backpropagated gradients using the RMSPROP algorithm with crossentropy as the objective function.3 In contrast to standard gradient descent, RMSPROP normalises the magnitude of the gradient components by dividing them by a root-mean-square moving average. We found this led to faster convergence. Other features of our training algorithm include the use of momentum to even out gradient oscillations, adaptive learning rates for each weight as well as adaptation of the global learning rate as a function of current training progress. The network is regularised with an ‘2 weight penalty. Good settings of the initial learning rate and the weight cost parameter (both around 0.001 in most experiments) were found by manual experi- mentation. Generally, we train our networks for 300 epochs, compute the validation error on a held-out set of some 10 % of the training data after each epoch and use the model that achieved the lowest validation error for testing. Since the source context features are very informative and it is comparatively more difficult to learn from the antecedents, the network sometimes had a tendency to overfit to the source features and disregard antecedent information. We found that this problem can be solved effectively by presenting a part of the training without any source features, forcing the network to learn from the information contained in the antecedents. In all experiments in this paper, we zero out all source features (input layers P, L1to L3 and R1 to R3) with a probability of 50 % in each training example. At test time, no information is zeroed out. Classification results with this network are shown in Table 3. We note that the accuracy has increased slightly for the TED test set and remains exactly the same for the news commentary corpus. However, a closer look on the results for individual classes reveals that the neural network makes better predictions for almost all classes. In terms of F-score, the only class that becomes slightly worse is the OTHER class for the news commentary corpus because of lower recall, indicating that the neural network classifier is less biased towards using the uninformative OTHER 3Our training procedure is greatly inspired by a series of online lectures held by Geoffrey Hinton in 2012 (https : //www . coursera. .org/course/neuralnets, 10 September 2013). 385 category. Recall for elle and elles increases considerably, but especially for elles it is still quite low. The increase in recall comes with some loss in precision, but the net effect on F-score is clearly positive. 6 Latent Anaphora Resolution Considering Figure 1 again, we note that the bilingual setting of our classification task adds some information not available to the monolingual anaphora resolver that can be helpful when determining the correct antecedent for a given pronoun. Knowing the gender of the translation of a pronoun limits the set of possible antecedents to those whose translation is morphologically compatible with the target language pronoun. We can exploit this fact to learn how to resolve anaphoric pronouns without requiring data with manually annotated anaphoric links. To achieve this, we extend our neural network with a component to predict the probability of each antecedent candidate to be the correct antecedent (Figure 4). The extended network is identical to the previous version except for the upper left part dealing with anaphoric link features. The only difference between the two networks is the fact that anaphora resolution is now performed by a part of our neural network itself instead of being done by an external module and provided to the classifier as an input. In this setup, we still use some parts of the BART toolkit to extract markables and compute features. However, we do not make use of the machine learning component in BART that makes the actual predictions. Since this is the only component trained on coreference-annotated data in a typical BART configuration, no coreference annotations are used anywhere in our system even though we continue to rely on the external anaphora resolver for preprocessing to avoid implementing our own markable and feature extractors and to make comparison easier. For each candidate markable identified by BART’s preprocessing pipeline, the anaphora resolution model receives as input a link feature vector (T) describing relevant aspects of the antecedent candidateanaphora pair. This feature vector is generated by the feature extraction machinery in BART and includes a standard feature set for coreference resolution partially based on work by Soon et al. (2001). We use the following feature extractors in BART, each of Figure 4: Neural network with latent anaphora resolution which can generate multiple features: Anaphora mention type Gender match Number match String match Alias feature (Soon et al., 2001) Appositive position feature (Soon et al., 2001) Semantic class (Soon et al., 2001) – – – – – – – Semantic class match Binary distance feature Antecedent is first mention in sentence Our baseline set of features was borrowed wholesale from a working coreference system and includes some features that are not relevant to the task at hand, e. g., features indicating that the anaphora is a pronoun, is not a named entity, etc. After removing all features that assume constant values in the training set when resolving antecedents for the set of pronouns we consider, we are left with a basic set of 37 anaphoric link features that are fed as inputs to our network. These features are exactly the same as those available to the anaphora resolution classifier in the BART system used in the previous section. Each training example for our network can have an arbitrary number of antecedent candidates, each of which is described by an antecedent word vector (A) and by an anaphoric link vector (T). The anaphoric link features are first mapped to a regular hidden layer with logistic sigmoid units (U). The activations of the hidden units are then mapped to a single value, which – – – 386 functions as an element in a softmax layer over all an- tecedent candidates (V). This softmax layer assigns a probability to each antecedent candidate, which we then use to compute a weighted average over the antecedent word vector, replacing the probabilities pi in Figures 2 and 3. At training time, the network’s anaphora resolution component is trained in exactly the same way as the rest of the network. The error signal from the embedding layer is backpropagated both to the weight matrix defining the antecedent word embedding and to the anaphora resolution subnetwork. Note that the number of weights in the network is the same for all training examples even though the number of antecedent candidates varies because all weights related to antecedent word features and anaphoric link features are shared between all antecedent candidates. One slightly uncommon feature of our neural network is that it contains an internal softmax layer to generate normalised probabilities over all possible antecedent candidates. Moreover, weights are shared between all antecedent candidates, so the inputs of our internal softmax layer share dependencies on the same weight variables. When computing derivatives with backpropagation, these shared dependen- cies must be taken into account. In particular, the outputs yi ofthe antecedent resolution layer are the result of a softmax applied to functions of some shared variables q: yi=∑kexepxp fi( fkq()q) (1) The derivatives of any yi with respect to q, which can be any of the weights in the anaphora resolution subnetwork, have dependencies on the derivatives of the other softmax inputs with respect to q: ∂∂yqi= yi ∂ f∂i(qq)−∑kyk∂ f∂k(qq)! (2) This makes the implementation of backpropagation for this part of the network somewhat more complicated, but in the case of our networks, it has no major impact on training time. Experimental results for this network are shown in Table 4. Compared with Table 3, we note that the overall accuracy is only very slightly lower for TED, and for the news commentaries it is actually better. When it comes to F-scores, the performance for elles improves by a small amount, while the effect on the other classes is a bit more mixed. Even where it gets worse, the differences are not dramatic considering that we eliminated a very knowledge-rich resource from the training process. This demonstrates that it is possible, in our classification task, to obtain good results without using any data manually annotated for anaphora and to rely entirely on unsupervised latent anaphora resolution. 7 Further Improvements The results presented in the preceding section represent a clear improvement over the ME classifiers in Table 2, even though the overall accuracy increased only slightly. Not only does our neural network classifier achieve better results on the classification task at hand without requiring an anaphora resolution classifier trained on manually annotated data, but it performs clearly better for the feminine categories that reflect minority choices requiring knowledge about the antecedents. Nevertheless, the performance is still not entirely satisfactory. By subjecting the output of our classifier on a development set to a manual error analysis, we found that a fairly large number oferrors belong to two error types: On the one hand, the preprocessing pipeline used to identify antecedent candidates does not always include the correct antecedent in the set presented to the neural network. Whenever this occurs, it is obvious that the classifier cannot possibly find 387 the correct antecedent. Out of 76 examples of the category elles that had been mistakenly predicted as ils, we found that 43 suffered from this problem. In other classes, the problem seems to be somewhat less common, but it still exists. On the other hand, in many cases (23 out of 76 for the category mentioned before) the anaphora resolution subnetwork does identify an antecedent manually recognised to belong to the right gender/number group, but still predicts an incorrect pronoun. This may indicate that the network has difficulties learning a correct gender/number representation for all words in the vocabulary. 7.1 Relaxing Markable Extraction The pipeline we use to extract potential antecedent candidates is borrowed from the BART anaphora resolution toolkit. BART uses a syntactic parser to identify noun phrases as markables. When extracting antecedent candidates for coreference prediction, it starts by considering a window consisting of the sentence in which the anaphoric pronoun is located and the two immediately preceding sentences. Markables in this window are checked for morphological compatibility in terms of gender and number with the anaphoric pronoun, and only compatible markables are extracted as antecedent candidates. If no compatible markables are found in the initial window, the window is successively enlarged one sentence at a time until at least one suitable markable is found. Our error analysis shows that this procedure misses some relevant markables both because the initial two-sentence extraction window is too small and because the morphological compatibility check incorrectly filters away some markables that should have been considered as candidates. By contrast, the extraction procedure does extract quite a number of first and second person noun phrases (I, we, you and their oblique forms) in the TED talks which are extremely unlikely to be the antecedent of a later occurrence of he, she, it or they. As a first step, we therefore adjust the extraction criteria to our task by increasing the initial extraction window to five sentences, excluding first and second person markables and removing the morphological compatibility requirement. The compatibility check is still used to control expansion of the extraction window, but it is no longer applied to filter the extracted markables. This increases the accuracy to 0.701 for TED and 0.602 for the news TED (Accuracy: 0.696) P R ce 0.618 0.722 elle 0.754 0.548 elles 0.737 0.340 il 0.718 0.629 ils 0.652 0.916 OTHER 0.741 0.682 F 0.666 0.635 0.465 0.670 0.761 0.711 News commentary (Accuracy: 0.597) ce elle elles il ils OTHER P 0.419 0.547 0.539 0.623 0.596 0.614 R 0.368 0.460 0.135 0.719 0.783 0.544 F 0.392 0.500 0.215 0.667 0.677 0.577 Table 4: Neural network classifier with latent anaphora resolution TED (Accuracy: 0.713) ce elle P 0.61 1 0.749 R 0.723 0.596 F 0.662 0.664 elles 0.602 0.616 il 0.733 0.638 ils 0.710 0.884 OTHER 0.760 0.704 News commentary (Accuracy: 0.626) ce elle elles il ils OTHER P 0.492 0.526 0.547 0.599 0.671 0.681 Table 5: Final classifier R 0.324 0.439 0.558 0.757 0.878 0.526 0.609 0.682 0.788 0.731 F 0.391 0.478 0.552 0.669 0.761 0.594 results commentaries, while the performance for elles im- proves to F-scores of 0.531 (TED; P 0.690, R 0.432) and 0.304 (News commentaries; P 0.444, R 0.231), respectively. Note that these and all the following results are not directly comparable to the ME baseline results in Table 2, since they include modifications and improvements to the training data extraction procedure that might possibly lead to benefits in the ME setting as well. 7.2 Adding Lexicon Knowledge In order to make it easier for the classifier to identify the gender and number properties of infrequent words, we extend the word vectors with features indicating possible morphological features for each word. In early experiments with ME classifiers, we found that our attempts to do proper gender and number tagging in French text did not improve classification performance noticeably, presumably because the annotation was too noisy. In more recent experiments, we just add features indicating all possible morphological interpretations of each word, rather than trying to disambiguate them. To do this, we look up the morphological annotations of the French words in the Lefff dictionary (Sagot et al., 2006) and intro- 388 duce a set of new binary features to indicate whether a particular reading of a word occurs in that dictionary. These features are then added to the one-hot representation of the antecedent words. Doing so improves the classifier accuracy to 0.71 1 (TED) and 0.604 (News commentaries), while the F-scores for elles reach 0.589 (TED; P 0.649, R 0.539) and 0.500 (News commentaries; P 0.545, R 0.462), respectively. 7.3 More Anaphoric Link Features Even though the modified antecedent candidate extraction with its larger context window and without the morphological filter results in better performance on both test sets, additional error analysis reveals that the classifiers has greater problems identifying the correct markable in this setting. One reason for this may be that the baseline anaphoric link feature set described above (Section 6) only includes two very rough binary distance features which indicate whether or not the anaphora and the antecedent candidate occur in the same or in immediately adjacent sentences. With the larger context window, this may be too unspecific. In our final experiment, we there- fore enable some additional features which are available in BART, but disabled in the baseline system: Distance in number of markables Distance in number of sentences Sentence distance, log-transformed Distance in number of words Part of speech of head word Most of these encode the distance between the anaphora and the antecedent candidate in more precise ways. Complete results for this final system are presented in Table 5. Including these additional features leads to another slight increase in accuracy for both corpora, with similar or increased classifier F-scores for most classes except elle in the news commentary condition. In particular, we should like to point out the performance of our benchmark classifier for elles, which suffered from extremely low recall in the first classifiers and approaches the performance ofthe other classes, with nearly balanced precision and recall, in this final system. Since elles is a low-frequency class and cannot be reliably predicted using source context alone, we interpret this as evidence that our final neural network classifier has incorporated some relevant knowledge about pronominal anaphora that the baseline ME clas– – – – – sifier and earlier versions of our network have no access to. This is particularly remarkable because no data manually annotated for coreference was used for training. 8 Related work Even though it was recognised years ago that the information contained in parallel corpora may provide valuable information for the improvement of anaphora resolution systems, there have not been many attempts to cash in on this insight. Mitkov and Barbu (2003) exploit parallel data in English and French to improve pronominal anaphora resolution by combining anaphora resolvers for the individual languages with handwritten rules to resolve conflicts between the output of the language-specific resolvers. Veselovská et al. (2012) apply a similar strategy to English-Czech data to resolve different uses of the pronoun it. Other work has used word alignments to project coreference annotations from one language to another with a view to training anaphora resolvers in the target language (Postolache et al., 2006; de Souza and Or˘ asan, 2011). Rahman and Ng (2012) instead use machine translation to translate their test 389 data into a language for which they have an anaphora resolver and then project the annotations back to the original language. Completely unsupervised monolingual anaphora resolution has been approached using, e. g., Markov logic (Poon and Domingos, 2008) and the Expectation-Maximisation algorithm (Cherry and Bergsma, 2005; Charniak and Elsner, 2009). To the best of our knowledge, the direct application of machine learning techniques to parallel data in a task related to anaphora resolution is novel in our work. Neural networks and deep learning techniques have recently gained some popularity in natural language processing. They have been applied to tasks such as language modelling (Bengio et al., 2003; Schwenk, 2007), translation modelling in statistical machine translation (Le et al., 2012), but also part-ofspeech tagging, chunking, named entity recognition and semantic role labelling (Collobert et al., 2011). In tasks related to anaphora resolution, standard feedforward neural networks have been tested as a classifier in an anaphora resolution system (Stuckardt, 2007), but the network design presented in our work is novel. 9 Conclusion In this paper, we have introduced cross-lingual pronoun prediction as an independent natural language processing task. Even though it is not an end-to-end task, pronoun prediction is interesting for several reasons. 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4 0.12489675 67 emnlp-2013-Easy Victories and Uphill Battles in Coreference Resolution

Author: Greg Durrett ; Dan Klein

Abstract: Classical coreference systems encode various syntactic, discourse, and semantic phenomena explicitly, using heterogenous features computed from hand-crafted heuristics. In contrast, we present a state-of-the-art coreference system that captures such phenomena implicitly, with a small number of homogeneous feature templates examining shallow properties of mentions. Surprisingly, our features are actually more effective than the corresponding hand-engineered ones at modeling these key linguistic phenomena, allowing us to win “easy victories” without crafted heuristics. These features are successful on syntax and discourse; however, they do not model semantic compatibility well, nor do we see gains from experiments with shallow semantic features from the literature, suggesting that this approach to semantics is an “uphill battle.” Nonetheless, our final system1 outperforms the Stanford system (Lee et al. (201 1), the winner of the CoNLL 2011 shared task) by 3.5% absolute on the CoNLL metric and outperforms the IMS system (Bj o¨rkelund and Farkas (2012), the best publicly available English coreference system) by 1.9% absolute.

5 0.10229257 108 emnlp-2013-Interpreting Anaphoric Shell Nouns using Antecedents of Cataphoric Shell Nouns as Training Data

Author: Varada Kolhatkar ; Heike Zinsmeister ; Graeme Hirst

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