Optimising Multiple Metrics with MERT

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Optimising Multiple Metrics with MERT

Optimisation in statistical machine translation is usually made toward the BLEU score, but this metric is questioned about its relevance to an human evaluation. Many other metrics exist but none of them are in perfect harmony with human evaluation. On the other hand, most evaluation campaigns use multiple metrics (BLEU, TER, METEOR, etc.). Statistical machine translation systems can be optimised for other metrics than BLEU, but usually the optimisation with other metrics tends to decrease the BLEU score, the main metric used in MT evaluation campaigns.

In this paper we extend the minimum error training tool of the popular Moses SMT toolkit with a scorer for the TER score, and any linear combination of the existing metrics. The TER scorer was reimplemented in C++ which results in a ten times faster execution than the reference java code.

We have performed experiments with two large-scale phrase-base SMT systems to show the benefit of the new options of the minimum error training in Moses. The first one translates from French into English (WMT 2011 evaluation). The second one was developed in the frame work of the DARPA Gale project to translate from Arabic to English in three different genres (news, web and transcribed broadcast news and conversations).

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