Extracting Paraphrases from a Parallel Corpus

Extracting Paraphrases from a Parallel Corpus Regina Barzilay and Kathleen R. McKeown Computer Science Department Columbia University 10027, New York, NY, USA � regina,kathy � @cs.columbia.edu Abstract While paraphrasing is critical both for interpretation and generation of natu- ral language, current systems use man- ual or semi-automatic methods to col- lect paraphrases. We present an un- supervised learning algorithm for iden- tification of paraphrases from a cor- pus of multiple English translations of the same source text. Our approach yields phrasal and single word lexical paraphrases as well as syntactic para- phrases. 1 Introduction Paraphrases are alternative ways to convey the same information. A method for the automatic acquisition of paraphrases has both practical and linguistic interest. From a practical point of view, diversity in expression presents a major challenge for many NLP applications. In multidocument summarization, identification of paraphrasing is required to find repetitive information in the in- put documents. In generation, paraphrasing is employed to create more varied and fluent text. Most current applications use manually collected paraphrases tailored to a specific application, or utilize existing lexical resources such as Word- Net (Miller et al., 1990) to identify paraphrases. However, the process of manually collecting para- phrases is time consuming, and moreover, the col- lection is not reusable in other applications. Ex- isting resources only include lexical paraphrases; they do not include phrasal or syntactically based paraphrases. From a linguistic point of view, questions concern the operative definition of paraphrases: what types of lexical relations and syntactic mechanisms can produce paraphrases? Many linguists (Halliday, 1985; de Beaugrande and Dressler, 1981) agree that paraphrases retain “ap- proximate conceptual equivalence”, and are not limited only to synonymy relations. But the ex- tent of interchangeability between phrases which form paraphrases is an open question (Dras, 1999). A corpus-based approach can provide in- sights on this question by revealing paraphrases that people use. This paper presents a corpus-based method for automatic extraction of paraphrases. We use a large collection of multiple parallel English trans- lations of novels1. This corpus provides many instances of paraphrasing, because translations preserve the meaning of the original source, but may use different words to convey the mean- ing. An example of parallel translations is shown in Figure 1. It contains two pairs of para- phrases: (“burst into tears”, “cried”) and (“com- fort”, “console”). Emma burst into tears and he tried to comfort her, say- ing things to make her smile. Emma cried, and he tried to console her, adorning his words with puns. Figure 1: Two English translations of the French sentence from Flaubert’s “Madame Bovary” Our method for paraphrase extraction builds upon methodology developed in Machine Trans- lation (MT). In MT, pairs of translated sentences from a bilingual corpus are aligned, and occur- rence patterns of words in two languages in the text are extracted and matched using correlation measures. However, our parallel corpus is far from the clean parallel corpora used in MT. The 1Foreign sources are not used in our experiment. rendition of a …

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