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Distributional Semantics Beyond Words: Supervised Learning of Analogy and Paraphrase

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Distributional Semantics Beyond Words: Supervised Learning of Analogy and Paraphrase Distributional Semantics Beyond Words: Supervised Learning of Analogy and Paraphrase Peter D. Turney National Research Council Canada Information and Communications Technologies Ottawa, Ontario, Canada, K1A 0R6 [email protected] Abstract Distributional semantics is highly effective for measuring the semantic similarity between individ- ual words. On a set of eighty multiple-choice syn- There have been several efforts to extend distributional semantics beyond individual onym questions from the test of English as a for- words, to measure the similarity of word pairs, eign language (TOEFL), a distributional approach phrases, and sentences (briefly, tuples; ordered recently achieved 100% accuracy (Bullinaria and sets of words, contiguous or noncontiguous). Levy, 2012). However, it has been difficult to extend One way to extend beyond words is to com- distributional semantics beyond individual words, to pare two tuples using a function that com- word pairs, phrases, and sentences. bines pairwise similarities between the com- Moving beyond individual words, there are vari- ponent words in the tuples. A strength of this approach is that it works with both rela- ous types of semantic similarity to consider. Here tional similarity (analogy) and compositional we focus on paraphrase and analogy. Paraphrase similarity (paraphrase). However, past work is similarity in the meaning of two pieces of text required hand-coding the combination func- (Androutsopoulos and Malakasiotis, 2010). Anal- tion for different tasks. The main contribution ogy is similarity in the semantic relations of two sets of this paper is that combination functions are of words (Turney, 2008a). generated by supervised learning. We achieve state-of-the-art results in measuring relational It is common to study paraphrase at the sentence similarity between word pairs (SAT analo- level (Androutsopoulos and Malakasiotis, 2010), but gies and SemEval 2012 Task 2) and measur- we prefer to concentrate on the simplest type of ing compositional similarity between noun- paraphrase, where a bigram paraphrases a unigram. modifier phrases and unigrams (multiple- For example, dog house is a paraphrase of kennel. In choice paraphrase questions). our experiments, we concentrate on noun-modifier bigrams and noun unigrams. Analogies map terms in one domain to terms in 1 Introduction another domain (Gentner, 1983). The familiar anal- ogy between the solar system and the Rutherford- Harris (1954) and Firth (1957) hypothesized that Bohr atomic model involves several terms from the words that appear in similar contexts tend to have domain of the solar system and the domain of the similar meanings. This hypothesis is the founda- atomic model (Turney, 2008a). tion for distributional semantics, in which words are The simplest type of analogy is proportional anal- represented by context vectors. The similarity of ogy, which involves two pairs of words (Turney, two words is calculated by comparing the two cor- 2006b). For example, the pair cook, raw is anal- h i responding context vectors (Lund et al., 1995; Lan- ogous to the pair decorate, plain . If we cook a h i dauer and Dumais, 1997; Turney and Pantel, 2010). thing, it is no longer raw; if we decorate a thing, it 353 Transactions of the Association for Computational Linguistics, 1 (2013) 353–366. Action Editor: Patrick Pantel. Submitted 5/2013; Revised 7/2013; Published 10/2013. c 2013 Association for Computational Linguistics. Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/tacl_a_00233 by guest on 24 September 2021 is no longer plain. The semantic relations between portional analogy cook, raw, decorate, plain is h i cook and raw are similar to the semantic relations labeled as a positive example. between decorate and plain. In the following exper- A quadruple is represented by a feature vector, iments, we focus on proportional analogies. composed of domain and function similarities from Erk (2013) distinguished four approaches to the dual-space model and other features based on extend distributional semantics beyond words: In corpus frequencies. SuperSim uses a support vector the first, a single vector space representation for a machine (Platt, 1998) to learn the probability that a phrase or sentence is computed from the represen- quadruple a, b, c, d consists of a word pair a, b h i h i tations of the individual words (Mitchell and Lap- and an analogous word pair c, d . The probability h i ata, 2010; Baroni and Zamparelli, 2010). In the sec- can be interpreted as the degree of relational similar- ond, two phrases or sentences are compared by com- ity between the two given word pairs. bining multiple pairwise similarity values (Socher et We also approach paraphrase as supervised tuple al., 2011; Turney, 2012). Third, weighted inference classification. To measure the compositional simi- rules integrate distributional similarity and formal larity beween an m-gram and an n-gram, we train logic (Garrette et al., 2011). Fourth, a single space the learning algorithm with (m + n)-tuples that are integrates formal logic and vectors (Clarke, 2012). positive and negative examples of paraphrases. Taking the second approach, Turney (2012) intro- SuperSim learns to estimate the probability that duced a dual-space model, with one space for mea- a triple a, b, c consists of a compositional bigram h i suring domain similarity (similarity of topic or field) ab and a synonymous unigram c. For instance, the and another for function similarity (similarity of role phrase fish tank is synonymous with aquarium; that or usage). Similarities beyond individual words are is, fish tank and aquarium have high compositional calculated by functions that combine domain and similarity. The triple fish, tank, aquarium is repre- h i function similarities of component words. sented using the same features that we used for anal- The dual-space model has been applied to mea- ogy. The probability of the triple can be interpreted suring compositional similarity (paraphrase recogni- as the degree of compositional similarity between tion) and relational similarity (analogy recognition). the given bigram and unigram. In experiments that tested for sensitivity to word We review related work in Section 2. The gen- order, the dual-space model performed significantly eral feature space for learning relations and compo- better than competing approaches (Turney, 2012). sitions is presented in Section 3. The experiments A limitation of past work with the dual-space with relational similarity are described in Section 4, model is that the combination functions were hand- and Section 5 reports the results with compositional coded. Our main contribution is to show how hand- similarity. Section 6 discusses the implications of coding can be eliminated with supervised learning. the results. We consider future work in Section 7 For ease of reference, we will call our approach and conclude in Section 8. SuperSim (supervised similarity). With no modifi- cation of SuperSim for the specific task (relational 2 Related Work similarity or compositional similarity), we achieve better results than previous hand-coded models. In SemEval 2012, Task 2 was concerned with mea- Compositional similarity (paraphrase) compares suring the degree of relational similarity between two contiguous phrases or sentences (n-grams), two word pairs (Jurgens et al., 2012) and Task 6 whereas relational similarity (analogy) does not (Agirre et al., 2012) examined the degree of seman- require contiguity. We use tuple to refer to both con- tic equivalence between two sentences. These two tiguous and noncontiguous word sequences. areas of research have been mostly independent, We approach analogy as a problem of supervised although Socher et al. (2012) and Turney (2012) tuple classification. To measure the relational sim- present unified perspectives on the two tasks. We ilarity between two word pairs, we train SuperSim first discuss some work on relational similarity, then with quadruples that are labeled as positive and neg- some work on compositional similarity, and lastly ative examples of analogies. For example, the pro- work that unifies the two types of similarity. 354 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/tacl_a_00233 by guest on 24 September 2021 2.1 Relational Similarity bigram ab is represented by c = a b, where c = a b . However, element-wise multiplica- LRA (latent relational analysis) measures rela- i i · i tion is commutative, so the bigrams ab and ba map tional similarity with a pair–pattern matrix (Turney, to the same vector c. In experiments that test for 2006b). Rows in the matrix correspond to word order sensitivity, element-wise multiplication per- pairs (a, b) and columns correspond to patterns that forms poorly (Turney, 2012). connect the pairs (“a for the b”) in a large cor- pus. This is a holistic (noncompositional) approach We can treat the bigram ab as a unit, as if it were to distributional similarity, since the word pairs are a single word, and construct a context vector for opaque wholes; the component words have no sep- ab from occurrences of ab in a large corpus. This arate representations. A compositional approach to holistic approach to representing bigrams performs analogy has a representation for each word, and a well when a limited set of bigrams is specified in word pair is represented by composing the represen- advance (before building the word–context matrix), tations for each member of the pair. Given a vocabu- but it does not scale up, because there
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