acl acl2010 acl2010-118 knowledge-graph by maker-knowledge-mining

118 acl-2010-Fine-Grained Tree-to-String Translation Rule Extraction


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Author: Xianchao Wu ; Takuya Matsuzaki ; Jun'ichi Tsujii

Abstract: Tree-to-string translation rules are widely used in linguistically syntax-based statistical machine translation systems. In this paper, we propose to use deep syntactic information for obtaining fine-grained translation rules. A head-driven phrase structure grammar (HPSG) parser is used to obtain the deep syntactic information, which includes a fine-grained description of the syntactic property and a semantic representation of a sentence. We extract fine-grained rules from aligned HPSG tree/forest-string pairs and use them in our tree-to-string and string-to-tree systems. Extensive experiments on largescale bidirectional Japanese-English trans- lations testified the effectiveness of our approach.

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 jp s t su j i Abstract Tree-to-string translation rules are widely used in linguistically syntax-based statistical machine translation systems. [sent-5, score-0.512]

2 In this paper, we propose to use deep syntactic information for obtaining fine-grained translation rules. [sent-6, score-0.318]

3 1 Introduction Tree-to-string translation rules are generic and applicable to numerous linguistically syntax-based Statistical Machine Translation (SMT) systems, such as string-to-tree translation (Galley et al. [sent-10, score-0.545]

4 (2004; 2006) are frequently used for extracting minimal and composed rules from aligned 1-best tree-string pairs. [sent-18, score-0.304]

5 Dealing with the parse error problem and rule sparseness problem, Mi and Huang (2008) replaced the 1-best parse tree with a packed forest which compactly encodes exponentially many parses for treeto-string rule extraction. [sent-19, score-0.817]

6 As will be testified by our experiments, we argue that the simple POS/phrasal tags are too coarse to reflect the accurate translation probabilities of the translation rules. [sent-22, score-0.46]

7 For example, as shown in Table 1, suppose a simple tree fragment “VBN(killed)” appears 6 times with “koroshita”, which is a Japanese translation of an active form of “killed”, and 4 times with “korosareta”, which is a Japanese translation of a passive form of “killed”. [sent-23, score-0.713]

8 Now, by attaching the voice information to “killed”, we are gaining a rule set that is more appropriate to reflect the real translation situations. [sent-28, score-0.391]

9 This motivates our proposal of using deep syntactic information to obtain a fine-grained translation rule set. [sent-29, score-0.472]

10 We name the information such as the voice of a verb in a tree fragment as deep syntactic information. [sent-30, score-0.384]

11 We use a head-driven phrase structure grammar (HPSG) parser to obtain the 1For example, “John has killed Mary. [sent-31, score-0.364]

12 We extract fine-grained translation rules from aligned HPSG tree/forest-string pairs. [sent-36, score-0.366]

13 We localize an HPSG tree/forest to make it segmentable at any nodes to fit the extraction algorithms described in (Galley et al. [sent-37, score-0.223]

14 We also propose a linear-time algorithm for extracting composed rules guided by predicate-argument structures. [sent-39, score-0.213]

15 The effectiveness of the rules are testified in our tree-to-string and string-to-tree systems, taking bidirectional Japanese-English translations as our test cases. [sent-40, score-0.254]

16 In Section 2, we briefly review the tree-to-string and string-totree translation frameworks, tree-to-string rule extraction algorithms, and rich syntactic information previously used for SMT. [sent-42, score-0.437]

17 The HPSG grammar and our proposal of fine-grained rule extraction algorithms are described in Section 3. [sent-43, score-0.235]

18 Section 4 gives the experiments for applying fine-grained translation rules to large-scale Japanese-English translation tasks. [sent-44, score-0.512]

19 1 Tree-to-string and string-to-tree translations Tree-to-string translation (Liu et al. [sent-47, score-0.197]

20 , 2006) first uses a parser to parse a source sentence into a 1-best tree and then searches for the best derivation that segments and converts the tree into a target string. [sent-49, score-0.343]

21 That is, giving a (bilingual) translation grammar and a source sentence, we are trying to construct a parse forest in the target language. [sent-54, score-0.446]

22 Consequently, the translation results can be collected from the leaves of the parse forest. [sent-55, score-0.246]

23 The English sentence is “John killed Mary” and the Japanese sentence is “jyon ha mari wo koroshita”, in which the function words “ha” and “wo” are not aligned with any English word. [sent-57, score-0.381]

24 f1J is a sentence of a foreign language other than English, Et is a 1-best parse tree of an English sentence E = e1I, and A = {(j, i)} is an alignment between the wo, ardnsd di nA AF = =an {d( jE,i. [sent-61, score-0.242]

25 The basic idea of GHKM algorithm is to decompose Et into a series of tree fragments, each of which will form a rule with its corresponding translation in the foreign language. [sent-62, score-0.544]

26 A is used as a constraint to guide the segmentation procedure, so that the root node of every tree fragment of Et exactly corresponds to a contiguous span on the foreign language side. [sent-63, score-0.46]

27 Based on this consideration, a frontier set (fs) is defined to be a set of nodes n in Et that satisfies the following constraint: fs = {n|span(n) ∩ comp span(n) = ϕ}. [sent-64, score-0.229]

28 With fs computed, rules are extracted through a depthfirst traversal of Et: we cut Et at all nodes in fs to form tree fragments and extract a rule for each fragment. [sent-78, score-0.762]

29 These extracted rules are called minimal rules (Galley et al. [sent-79, score-0.276]

30 For example, the 1best tree (with gray nodes) in Figure 2 is cut into 7 pieces, each of which corresponds to the tree fragment in a rule (bottom-left corner of the figure). [sent-81, score-0.582]

31 In order to include richer context information and account for multiple interpretations of unaligned words of foreign language, minimal rules which share adjacent tree fragments are connected together to form composed rules (Galley et al. [sent-82, score-0.595]

32 (2006) constructed a derivation-forest, in which composed rules were generated, unaligned words of foreign language were consistently attached, and the translation probabilities of rules were estimated by using ExpectationMaximization (EM) (Dempster et al. [sent-85, score-0.562]

33 For example, by combining the minimal rules of 1, 4, and 5, we obtain a composed rule, as shown in the bottom-right corner of Figure 2. [sent-87, score-0.267]

34 Considering the parse error problem in the 1-best or k-best parse trees, Mi and Huang (2008) extracted tree-to-string translation rules from aligned packed forest-string pairs. [sent-88, score-0.577]

35 A forest compactly encodes exponentially many trees 327 rather than the 1-best tree used by Galley et al. [sent-89, score-0.298]

36 Two problems were managed to be tackled during extracting rules from an aligned forest-string pair: where to cut and how to cut. [sent-91, score-0.228]

37 Equation 1 was used again to compute a frontier node set to determine where to cut the packed forest into a number of tree-fragments. [sent-92, score-0.427]

38 The difference with tree-based rule extraction is that the nodes in a packed forest (which is a hypergraph) now are hypernodes, which can take a set of incoming hyperedges. [sent-93, score-0.541]

39 Then, by limiting each frag- ment to be a tree and whose root/leaf hypernodes all appearing in the frontier set, the packed forest can be segmented properly into a set of tree fragments, each of which can be used to generate a tree-to-string translation rule. [sent-94, score-0.806]

40 3 Rich syntactic information for SMT Before describing our approaches of applying deep syntactic information yielded by an HPSG parser for fine-grained rule extraction, we would like to briefly review what kinds of deep syntactic information have been employed for SMT. [sent-96, score-0.461]

41 (2007) also reported a significant improvement for Dutch-English translation by applying CCG supertags at a word level to a factorized SMT system (Koehn et al. [sent-104, score-0.279]

42 The major differences are that, we use a larger feature set (Table 2) including the supertags for fine-grained tree-to-string rule extraction, rather than string-to-string translation (Hassan et al. [sent-115, score-0.433]

43 In contrast, the fine-grained tree-to-string translation rule extraction approaches in this paper are totally data-driven, and easily applicable to numerous language pairs by taking English as the source or target language. [sent-121, score-0.431]

44 3 Fine-grained rule extraction We now introduce the deep syntactic information generated by an HPSG parser and then describe our approaches for fine-grained tree-tostring rule extraction. [sent-122, score-0.476]

45 Also, we propose a linear-time composed rule extraction algorithm by making use of predicate-argument structures. [sent-125, score-0.269]

46 First, we can carefully control the condition of the application of a translation rule by exploiting the fine-grained syntactic 2http://www. [sent-136, score-0.39]

47 description in the English parse tree/forest, as well as those in the translation rules. [sent-142, score-0.246]

48 We expect that extraction of translation rules based on such semantically-connected subtrees will give a compact and effective set of translation rules. [sent-144, score-0.559]

49 html kilA R G 21 JMoahrny ignAoRrAeGRG21factShewARAanGRAtG2R2G1dIAisRpGut1e Figure 3: Predicate argument structures for the sentences of “John killed Mary” and “She ignored the fact that I wanted to dispute”. [sent-162, score-0.33]

50 Figure 3 shows the PAS of the example sentence in Figure 2, “John killed Mary”, and a more complex PAS for another sentence, “She ignored the fact that I wanted to dispute”, which is adopted from (Miyao et al. [sent-165, score-0.33]

51 2 Localize HPSG forest Our fine-grained translation rule extraction algorithm is sketched in Algorithm 1. [sent-170, score-0.564]

52 Considering that a parse tree is a trivial packed forest, we only use the term forest to expand our discussion, hereafter. [sent-171, score-0.46]

53 However, the three nodes are not included in one (minimal) translation rule. [sent-175, score-0.258]

54 , 2006) 6: else if Ef is an HPSG forest then ′′ ′ 7: 8: Ef′ = localize Forest(Ef); R2 = forest based rule extraction(E′f, F, A) ◃ AlgorRithm 1in (Mi and Huang, 2008) 9: end if the identifier of the daughter node as the values. [sent-183, score-0.782]

55 A pure syntactic-based HPSG forest without any pointer-valued features can be yielded through this pre-processing for the consequent execution of the extraction algorithms (Galley et al. [sent-186, score-0.213]

56 3 Predicate-argument structures In order to extract translation rules from PASs, we want to localize a predicate word and its arguments into one tree fragment. [sent-189, score-0.605]

57 For example, in Figure 2, we can use a tree fragment which takes c0 as its root node and c1, t1, and c5 on its yield (= leaf nodes of a tree fragment) to cover “killed” and its subject and direct object arguments. [sent-190, score-0.583]

58 We define this kind of tree fragment to be a minimum covering tree. [sent-191, score-0.349]

59 For example, the minimum covering tree of {t1, c1, c5} is shown in the bottom-right corner o {ft Figure 2}. [sent-192, score-0.299]

60 Tsh seh dwefnin iinti tohne supplies us a linear-time algorithm to directly find the tree fragment that covers a PAS during both rule extracting and rule matching when decoding an HPSG tree. [sent-193, score-0.639]

61 Algorithm 2 PASR extraction Input: HPSG tree Et, foreign sentence F, and alignment A Output: a PAS-based rule set R 1: R = {} 12:: fRor = =n {od}e n ∈ Leaves(Et) do 23:: i fn Oodpeen n( ∈n. [sent-194, score-0.394]

62 ARGs) 5: if root and leaf nodes of Tc are in fs then 6: generate a rule r using fragment Tc 7: R. [sent-196, score-0.476]

63 Taking a minimum covering tree as the tree fragment, we can easily build a tree-to-string translation rule that reflects the semantic dependency of a PAS. [sent-199, score-0.741]

64 The algorithm of PAS-based rule (PASR) extraction is sketched in Algorithm 2. [sent-200, score-0.201]

65 We extract PAS-based rules through one-time traversal of the leaf nodes in Et (line 2). [sent-204, score-0.275]

66 For each leaf node n, we extract a minimum covering tree Tc if n contains at least one argument. [sent-205, score-0.39]

67 Based on Tc, we can easily build a tree-to-string translation rule by further completing the right-hand-side string by sorting the spans of Tc’s leaf nodes, lexicalizing the terminal node’s span(s), and assigning a variable to each non-terminal node’s span. [sent-208, score-0.432]

68 Maximum likelihood estimation is used to calculate the translation probabilities of each rule. [sent-209, score-0.197]

69 Both models use a phrase translation table (PTT), an HPSG tree-based rule set (TRS), and a PAS-based rule set (PRS). [sent-214, score-0.505]

70 Since the three rule sets are independently extracted and estimated, we 330 use Minimum Error Rate Training (MERT) (Och, 2003) to tune the weights of the features from the three rule sets on the development set. [sent-215, score-0.308]

71 ∏r∈d This equation reflects that the translation rules in one d come from three sets. [sent-218, score-0.315]

72 , 2009b), it is appealing to combine these rule sets together in one decoder because PTT provides excellent rule coverages while TRS and PRS offer linguistically motivated phrase selections and nonlocal reorderings. [sent-220, score-0.337]

73 , 2006) and inversely binarize all translation rules into Chomsky Normal Forms that contain at most two variables and can be incrementally scored by LM. [sent-225, score-0.315]

74 The string-to-tree decoder searches for the optimal derivation d∗ that parses a Japanese string F into a packed forest of the set of all possible derivations D: d∗ =argmax{λ1logpLM(τ(d)) + λ3g(d) + log s(d|F)}. [sent-229, score-0.338]

75 2 Decoding algorithms In our translation models, we have made use of three kinds of translation rule sets which are trained separately. [sent-233, score-0.574]

76 , 2009b) for mixing different types of translation rules within one derivation. [sent-235, score-0.315]

77 Recall the definition of minimum covering tree, which supports a faster way to retrieve available rules from PRS without generating all the subtrees. [sent-239, score-0.244]

78 That is, when node n fortunately to be the root of some minimum covering tree(s), we use the tree(s) to seek available PAS-based rules in PRS. [sent-240, score-0.358]

79 For example, suppose we are decoding an HPSG tree (with gray nodes) shown in Figure 2. [sent-244, score-0.213]

80 At t1, we can extract its minimum covering tree with the root node to be c0, then take this tree fragment as the key to retrieve PRS, and consequently put c0 and the available rules in the hash-table. [sent-245, score-0.713]

81 We modified this parser to output a packed forest for each English sentence. [sent-270, score-0.279]

82 , 2007) on the training set to obtain a phrase- aligned parallel corpus, from which bidirectional phrase translation tables were estimated. [sent-272, score-0.318]

83 We evaluated the translation quality using the case-insensitive BLEU-4 metric (Papineni et al. [sent-274, score-0.197]

84 3M translation rules from the training set for the 4K English and Japanese sentences in the development and test sets. [sent-281, score-0.315]

85 The corpus can be conditionally obtained from NTCIR-7 patent translation workshop homepage: http://research. [sent-286, score-0.197]

86 200 for English-to-Japanese translation and 500 for Japanese-to-English translation. [sent-309, score-0.197]

87 Table 5 reports the BLEU-4 scores achieved by decoding the test set making use of Joshua and our systems (t2s = tree-to-string and s2t = string-totree) under numerous rule sets. [sent-314, score-0.268]

88 We take C3S and FS as approximations of CFG-based translation rules. [sent-317, score-0.197]

89 The decoding time (seconds per sentence) of tree-to-string translation is listed as well. [sent-352, score-0.278]

90 Furthermore, in Table 5, the decoding time (sec- onds per sentence) of tree-to-string translation by using PTT+PRS is more than 86 times faster than using the other tree-to-string rule sets. [sent-356, score-0.432]

91 This suggests that the direct generation of minimum covering trees for rule matching is extremely faster than generating all subtrees of a tree node. [sent-357, score-0.412]

92 5 Conclusion We have proposed approaches of using deep syntactic information for extracting fine-grained treeto-string translation rules from aligned HPSG forest-string pairs. [sent-368, score-0.514]

93 , 2006; Mi and Huang, 2008) to HPSG forests and a linear-time algorithm for extracting composed rules from predicate-argument structures. [sent-370, score-0.213]

94 We applied our fine-grained translation rules to a tree-to-string system and an Hiero-style string-totree system. [sent-371, score-0.315]

95 We argue the fine-grained translation rules are generic and applicable to many syntax-based SMT frameworks such as the forest-to-string model (Mi et al. [sent-373, score-0.315]

96 Furthermore, it will be interesting to extract fine-grained tree-to-tree translation rules by integrating deep syntactic information in the source and/or target language side(s). [sent-375, score-0.436]

97 These treeto-tree rules are applicable for forest-to-tree translation models (Liu et al. [sent-376, score-0.315]

98 Scalable inference and training of context-rich syntactic translation models. [sent-416, score-0.236]

99 Towards hybrid quality-oriented machine translation - on linguistics and probabilities in mt. [sent-479, score-0.197]

100 Z-MERT: A fully configurable open source tool for minimum error rate training of machine translation systems. [sent-508, score-0.255]


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