What types of word alignment improve statistical machine translation?

Abstract

In most statistical machine translation (SMT) systems, bilingual segments are extracted via word alignment. However, there is a need for systematic study as to what alignment characteristics can benefit MT under specific experimental settings such as the type of MT system, the language pair or the type or size of the corpus. In this paper we perform, in each of these experimental settings, a statistical analysis of the data and study the sample correlation coefficients between a number of alignment or phrase table characteristics and variables such as the phrase table size, the number of untranslated words or the BLEU score. We report results for two different SMT systems (a phrase-based and an n-gram-based system) on Chinese-to-English FBIS and BTEC data, and Spanish-to-English European Parliament data. We find that the alignment characteristics which help in translation greatly depend on the MT system and on the corpus size. We give alignment hints to improve BLEU score, depending on the SMT system used and the type of corpus. For example, for phrase-based SMT, dense alignments are required with larger corpora, especially on the target side, while with smaller corpora, more precise, sparser alignments are better, especially on the source side. Avoiding some long-distance crossing links may also improve BLEU score with small corpora. We take these conclusions into account to modify two types of alignment systems, and get 1 to 1.6 % relative improvements in BLEU score on two held-out corpora, although the improved system is different in each corpus.