acl acl2013 acl2013-222 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Aliaksei Severyn ; Massimo Nicosia ; Alessandro Moschitti
Abstract: Measuring semantic textual similarity (STS) is at the cornerstone of many NLP applications. Different from the majority of approaches, where a large number of pairwise similarity features are used to represent a text pair, our model features the following: (i) it directly encodes input texts into relational syntactic structures; (ii) relies on tree kernels to handle feature engineering automatically; (iii) combines both structural and feature vector representations in a single scoring model, i.e., in Support Vector Regression (SVR); and (iv) delivers significant improvement over the best STS systems.
Reference: text
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1 CnRiIc, Qatar ,Fmoounscdahtiiotn,t Dio}@hda, Qatar amo s chitt i qf . [sent-5, score-0.057]
2 qa @ Abstract Measuring semantic textual similarity (STS) is at the cornerstone of many NLP applications. [sent-7, score-0.22]
3 1 Introduction In STS the goal is to learn a scoring model that given a pair of two short texts returns a similar- ity score that correlates with human judgement. [sent-11, score-0.139]
4 Hence, the key aspect of having an accurate STS framework is the design of features that can adequately represent various aspects of the similarity between texts, e. [sent-12, score-0.162]
5 The majority of approaches treat input text pairs as feature vectors where each feature is a score corresponding to a certain type of similarity. [sent-15, score-0.28]
6 This approach is conceptually easy to implement and the STS shared task at SemEval 2012 (Agirre et al. [sent-16, score-0.051]
7 , a number of features encoding similarity of an input text pair were combined in a single scoring model, e. [sent-19, score-0.35]
8 Nevertheless, one limitation of using only similarity features to represent a text pair is that of low representation power. [sent-22, score-0.296]
9 The novelty of our approach is that we treat the input text pairs as structural objects and rely on the power of kernel learning to extract relevant structures. [sent-23, score-0.661]
10 To link the documents in a pair we mark the nodes in the related structures with a special rela- tional tag. [sent-24, score-0.212]
11 This way effective structural relational patterns are implicitly encoded in the trees and can be automatically learned by the kernel-based machines. [sent-25, score-0.604]
12 We combine our relational structural model with the features from two best systems of STS-2012. [sent-26, score-0.543]
13 Finally, we use the approach of classifier stacking to combine several structural models into the feature vector representation. [sent-27, score-0.493]
14 2 Structural Relational Similarity The approach of relating pairs of input structures by learning predictable syntactic transformations has shown to deliver state-of-the-art results in question answering, recognizing textual entailment, and paraphrase detection, e. [sent-31, score-0.357]
15 applying quasi-synchronous grammar formalism and variations of tree edit distance alignments, to extract syntactic patterns relating pairs of input structures. [sent-37, score-0.294]
16 Our approach is conceptually simpler, as it regards the problem within the kernel learning framework, where we first encode salient syntactic/semantic proper714 Proce dingSsof oifa, th Beu 5l1gsarti Aan,An u aglu Mste 4e-ti9n2g 0 o1f3 t. [sent-38, score-0.255]
17 c A2s0s1o3ci Aatsiosonc fioartio Cno fmorpu Ctoamtiopnuatalt Lioin gauli Lsitnicgsu,i psatgices 714–718, ties of the input text pairs into tree structures and rely on tree kernels to automatically generate rich feature spaces. [sent-40, score-0.728]
18 , 2007; Moschitti and Quarteroni, 2008), in textual entailment recognition, e. [sent-44, score-0.103]
19 , (Moschitti and Zanzotto, 2007), and more in general in relational text categorization (Moschitti, 2008; Severyn and Moschitti, 2012). [sent-46, score-0.219]
20 In this section we describe: (i) a kernel framework to combine structural and vector models; (ii) structural kernels to handle feature engineering; and (iii) suitable structural representations for relational learning. [sent-47, score-1.688]
21 1 Structural Kernel Learning In supervised learning, given labeled data {(xx x i, yy i)}in=1, the goal is to estimate a decision f{u( xxx nction) h(xx x ) = yy that maps input examples to their targets. [sent-49, score-0.25]
22 A conventional approach is to represent a pair of texts as a set of similarity features {fi}, s. [sent-50, score-0.255]
23 the predictions are computed as h(xx x ) = w ww ·x x x = Piwifi, wionhsere a w ww ciso mthpeu mtedod aesl weight vweww ct ·o xxx r. [sent-52, score-0.215]
24 HePnce, the learning problem boils down to estimating individual weights of each of the similarity features fi. [sent-53, score-0.162]
25 One downside of such approach is that a great deal of similarity information encoded in a given text pair is lost when modeled by single real-valued scores. [sent-54, score-0.259]
26 A more versatile approach in terms of the input representation relies on kernels. [sent-55, score-0.137]
27 , SVM, the prediction function for Pa test input xx takes on the following form h(xx x ) = Pi αiyiK(x xx , xxi), where αi are the model parametPers estimated from the training data, yi are target vaPriables, xx i are support vectors, and K(·, ·) is a kernel function. [sent-58, score-1.12]
28 a To encode both structural representation and similarity feature vectors of a given text pair in a single model we define each document in a pair to be composed of a tree and a vector: htt t , vv i . [sent-59, score-0.97]
29 Each of the ker- ered: K( xx xi, xx xj) = nel computations K can be broken down into the following: = + Kfvec(v vv (1), vv v(2)), where KTK computes a structural kernel and Kfvec is a kernel over feature vec- K(x xx (1),x xx(2)) KTK( tt t(1), t tt(2)) tors, e. [sent-61, score-1.782]
30 Further in the text we refer to structural tree kernel models as TK and explicit feature vector representation as fve c. [sent-64, score-0.816]
31 Having defined a way to jointly model text pairs using structural TK representations along with the similarity features fvec, we next briefly review tree kernels and our relational structures. [sent-65, score-1.149]
32 2 Tree Kernels We use tree structures as our base representation since they provide sufficient flexibility in representation and allow for easier feature extraction than, for example, graph structures. [sent-67, score-0.372]
33 Hence, we rely on tree kernels to compute KTK(·, ·). [sent-68, score-0.328]
34 Given two trees it evaluates the number of s(u·,b·s)t. [sent-69, score-0.055]
35 Different TK functions are characterized by alternative fragment definitions. [sent-73, score-0.03]
36 In particular, we focus on the Syntactic Tree kernel (STK) (Collins and Duffy, 2002) and a Partial Tree Kernel (PTK) (Moschitti, 2006). [sent-74, score-0.163]
37 STK generates all possible substructures rooted in each node of the tree with the constraint that pro- duction rules can not be broken (i. [sent-75, score-0.201]
38 , any node in a tree fragment must include either all or none of its children). [sent-77, score-0.16]
39 PTK can be more effectively applied to both constituency and dependency parse trees. [sent-78, score-0.246]
40 It generalizes STK as the fragments it generates can contain any subset of nodes, i. [sent-79, score-0.052]
41 , PTK allows for breaking the production rules and generating an extremely rich feature space, which results in higher generalization ability. [sent-81, score-0.058]
42 3 Structural representations In this paper, we define simple-to-build relational structures based on: (i) a shallow syntactic tree, (ii) constituency, (iii) dependency and (iv) phrasedependency trees. [sent-83, score-0.597]
43 Shallow tree is a two-level syntactic hierarchy built from word lemmas (leaves), part-of-speech tags (preterminals) that are further organized into chunks. [sent-84, score-0.251]
44 It was shown to significantly outperform feature vector baselines for modeling relationships between question answer pairs (Severyn and Moschitti, 2012). [sent-85, score-0.182]
45 While shallow syntactic pars- ing is very fast, here we consider using constituency structures as a potentially richer source of syntactic/semantic information. [sent-87, score-0.405]
46 We propose to use dependency relations between words to derive an alternative structural representation. [sent-89, score-0.392]
47 In particular, de715 Figure 1: A phrase dependency-based structural representation of a text pair (s1, s2): A woman with a knife is slicing a pepper (s1) vs. [sent-90, score-0.576]
48 A women slicing green pepper (s2) with a high semantic similarity (human judgement score 4. [sent-91, score-0.281]
49 Related tree fragments are linked with a REL tag. [sent-94, score-0.182]
50 This reordering of the nodes helps to avoid the situation where nodes with words tend to form long chains. [sent-96, score-0.102]
51 We also plug part-of-speech tags between the word nodes and nodes carrying their grammatical role. [sent-98, score-0.193]
52 We explore a phrase- dependency tree similar to the one defined in (Wu et al. [sent-100, score-0.208]
53 It represents an alternative structure derived from the dependency tree, where the dependency relations between words belonging to the same phrase (chunk) are collapsed in a unified node. [sent-102, score-0.187]
54 , 2009), the collapsed nodes are stored as a shallow subtree rooted at the unified node. [sent-104, score-0.215]
55 This node organization is particularly suitable for PTK that effectively runs a sequence kernel on the tree fragments inside each chunk subtree. [sent-105, score-0.382]
56 Fig 1gives an example of our variation of a phrase dependency tree. [sent-106, score-0.078]
57 As a final consideration, if a document contains multiple sentences they are merged in a single tree with a common root. [sent-107, score-0.13]
58 To encode the structural relationships between documents in a pair a special REL tag is used to link the related structures. [sent-108, score-0.462]
59 Along with the direct representation of input text pairs as structural objects our framework is also capable of encoding pairwise similarity feature vectors (fvec), which we describe below. [sent-111, score-0.764]
60 (base) We adopt similarity features from two best performing systems of STS-2012, which were publicly released1 : namely, the Takelab2 system (Sˇari´ c et al. [sent-113, score-0.162]
61 Both systems represent input texts with similarity features combining multiple text similarity measures of varying complexity. [sent-116, score-0.395]
62 It also includes features derived from Explicit Semantic Analysis (Gabrilovich and Markovitch, 2007) and aggregation of word similarity based on lexical-semantic resources, e. [sent-118, score-0.162]
63 Takelab (T) includes n-gram matching of varying size, weighted word matching, length difference, WordNet similarity and vector space similarity where pairs of input sentences are mapped into Latent Semantic Analysis (LSA) space. [sent-122, score-0.379]
64 The features are computed over several sentence representations where stop words are removed and/or lemmas are used in place of raw tokens. [sent-123, score-0.174]
65 com/p/dkpro-similarityasl/wiki/SemEval2013 716 Altun, 2006), (iii) named entities, (iv) dependency triplets, and (v) PTK syntactic similarity scores computed between documents in a pair, where as input representations we use raw dependency and constituency trees. [sent-132, score-0.632]
66 To integrate multiple TK representations into a single model we apply a classifier stacking approach (Fast and Jensen, 2008). [sent-136, score-0.167]
67 Each of the learned TK models is used to generate predictions which are then plugged as features into the final fvec representation, s. [sent-137, score-0.196]
68 the final model uses only explicit feature vector representation. [sent-139, score-0.138]
69 We use the entire training data to obtain a single model for making predictions on each test set. [sent-147, score-0.03]
70 To encode TK models along with the similarity feature vectors into a single regression scoring model, we use an SVR framework implemented in SVM-Light-TK4. [sent-149, score-0.36]
71 We use the following parameter settings -t 5 -F 1 -W A -C +, which specifies a combination of trees and feature vectors (-C +), STK over trees (-F 1) (-F 3 for PTK) computed in all-vs-all mode (-W A) and polynomial kernel of degree 3 for the feature vector (active by default). [sent-150, score-0.516]
72 2 Results Table 1 summarizes the results of combining TK models with a strong feature vector model. [sent-157, score-0.103]
73 5 Conclusions and Future Work We have presented an approach where text pairs are directly treated as structural objects. [sent-162, score-0.377]
74 This provides a much richer representation for the learning algorithm to extract useful syntactic and shallow semantic patterns. [sent-163, score-0.211]
75 We have provided an extensive experimental study of four different structural representations, e. [sent-164, score-0.314]
76 shallow, constituency, dependency and phrase-dependency trees using STK and PTK. [sent-166, score-0.133]
77 via stacking; (iii) to our knowledge, this work is the first to apply structural kernels and combinations in a regression setting; and (iv) our model achieves the state of the art in STS largely improving the best previous systems. [sent-169, score-0.553]
78 Our structural learning approach to STS is conceptually simple and does not re- quire additional linguistic sources other than offthe-shelf syntactic parsers. [sent-170, score-0.407]
79 It is particularly suitable for NLP tasks where the input domain comes 5we also report the results for a concatenation of all five test sets (ALL) 717 Table 1: Results on STS-2012. [sent-171, score-0.051]
80 First set of experiments studies the combination of fvec models from UKP (U), Takelab (T) and (A). [sent-172, score-0.128]
81 Next we show results for four structural representations: shallow (S), constituency (C), dependency (D) and phrase-dependency (P) trees with STK and PTK; next row set demonstrates the necessity of relational linking for two best structures, i. [sent-173, score-0.903]
82 C and D (empty circle denotes a structures with no relational linking. [sent-175, score-0.289]
83 ); finally, domain adaptation via bags of features (B) of the entire pair and (M) manually encoded dataset type show the state of the art results. [sent-176, score-0.211]
84 Semeval-2012 task 6: A pilot on semantic textual similarity. [sent-183, score-0.096]
85 Ukp: Computing semantic textual similarity by combining multiple content similarity measures. [sent-187, score-0.344]
86 Broad-coverage sense disambiguation and information extraction with a supersense sequence tagger. [sent-191, score-0.049]
87 Tree edit models for recognizing textual entailments, para- phrases, and answers to questions. [sent-209, score-0.095]
88 Fast and effective kernels for relational learning from texts. [sent-217, score-0.389]
89 Exploiting syntactic and shallow semantic kernels for question/answer classification. [sent-221, score-0.366]
90 Efficient convolution kernels for dependency and constituent syntactic trees. [sent-225, score-0.318]
91 Kernel methods, syntax and semantics for relational text categorization. [sent-229, score-0.219]
92 Probabilistic tree-edit models with structured latent variables for textual entailment and question answering. [sent-242, score-0.103]
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