Word to word alignment strategies

Word to word alignment strategies J¨org Tiedemann Department of Linguistics and Philology Uppsala University Uppsala/Sweden joerg@stp.ling.uu.se Abstract Word alignment is a challenging task aim- ing at the identification of translational re- lations between words and multi-word units in parallel corpora. Many alignment strate- gies are based on links between single words. Different strategies can be used to find the optimal word alignment using such one-to- one word links including relations between multi-word units. In this paper seven algo- rithms are compared using a word alignment approach based on association clues and an English-Swedish bitext together with a handcrafted reference alignment used for evaluation. 1 Introduction Word alignment is the task of identifying trans- lational relations between words in parallel cor- pora with the aim of re-using them in natu- ral language processing. Typical applications that make use of word alignment techniques are machine translation and multi-lingual lex- icography. Several approaches have been pro- posed for the automatic alignment of words and phrases using statistical techniques and align- ment heuristics, e.g. (Brown et al., 1993; Vogel et al., 1996; Garc´ıa-Varea et al., 2002; Ahren- berg et al., 1998; Tiedemann, 1999; Tufis and Barbu, 2002; Melamed, 2000). Word align- ment usually includes links between so-called multi-word units (MWUs) in cases where lex- ical items cannot be split into separated words with appropriate translations in another lan- guage. See for example the alignment between an English sentence and a Swedish sentence il- lustrated in figure 1. There are MWUs in both languages aligned to corresponding translations in the other language. The Swedish compound “mittplatsen” corresponds to three words in En- glish (“the middle seat”) and the English verb “dislike” is translated into a Swedish particle verb “tycker om” (English: like) that has been negated using “inte”. Most approaches model Jag tar mittplatsen, vilket jag inte tycker om, men det gör mig inte så mycket. I take the middle seat, which I dislike, but I am not really put out. Figure 1: A word alignment example from Saul Bellow “To Jerusalem and back: a personal ac- count” (Bellow, 1976) and its Swedish transla- tion (Bellow, 1977) (the Bellow corpus). word alignment as links between words in the source language and words in the target lan- guage as indicated by the arrows in figure 1. However, in cases like the English expression “I am not really put out” which corresponds to the Swedish expression “det g¨or mig inte s˚a my- cket” there is no proper way of connecting single words with each other in order to express this relation. In some approaches such relations are constructed in form of an exhaustive set of links between all word pairs included in both expres- sions (Melamed, 1998; Mihalcea and Pedersen, 2003). In other approaches complex expressions are identified in a pre-processing step in order to handle them as complex units in the same manner as single words in alignment (Smadja et al., 1996; Ahrenberg et al., 1998; Tiedemann, 1999). The one-to-one word linking approach seems to be …

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