Hybrid Machine Translation by Combining Multiple Machine Translation Systems

Abstract

This thesis aims to research methods and develop tools that allow to successfully combine output from various machine translation (MT) systems so that the overall translation quality of the source text would increase. Applicability of the developed methods for small, morphologically rich and under-resourced languages is evaluated, especially Latvian and Estonian. Existing methods have been analysed, and several combinations of methods have been proposed. The proposed methods have been implemented and evaluated using automatic and human evaluation. During this research novel methods have been created that structure source language sentences into linguistically motivated fragments and combine them using a character level neural language model; combine neural machine translation output by employing source-translation attention alignments; use a multi-pass approach to produce additional incrementally improving training data. The key results of this research are new state-of-the-art machine translation systems for English ↔ Estonian; approaches for utilising neural MT generated attention alignments for MT combination and comprehension of resulting translations; MT combination systems for combining output from English → Latvian statistical MT. A practical application of the methods is implemented and described.