Adaptation of machine translation for multilingual information retrieval in the medical domain

Objective. We investigate machine translation (MT) of user search queries in the context of cross-lingual information retrieval (IR) in the medical domain. The main focus is on techniques to adapt MT to increase translation quality; however, we also explore MT adaptation to improve eectiveness of cross-lingual IR. Methods and Data. Our MT system is Moses, a state-of-the-art phrase-based statistical machine translation system. The IR system is based on the BM25 retrieval model implemented in the Lucene search engine. The MT techniques employed in this work include in-domain training and tuning, intelligent training data selection, optimization of phrase table configuration, compound splitting, and exploiting synonyms as translation variants. The IR methods include morphological normalization and using multiple translation variants for query expansion. The experiments are performed and thoroughly evaluated on three language pairs: Czech–English, German–English, and French–English. MT quality is evaluated on data sets created within the Khresmoi project and IR eectiveness is tested on the CLEF eHealth 2013 data sets. Results. The search query translation results achieved in our experiments are outstanding – our systems outperform not only our strong baselines, but also Google Translate and Microsoft Bing Translator in direct comparison carried out on all the language pairs. The baseline BLEU scores increased from 26.59 to 41.45 for Czech–English, from 23.03 to 40.82 for German–English, and from 32.67 to 40.82 for French–English. This is a 55% improvement on average. In terms of the IR performance on this particular test collection, a significant improvement over the baseline is achieved only for French–English. For Czech–English and German–English, the increased MT quality does not lead to better IR results. Conclusions. Most of the MT techniques employed in our experiments improve MT of medical search queries. Especially the intelligent training data selection proves to be very successful for domain adaptation of MT. Certain improvements are also obtained from German compound splitting on the source language side. Translation quality, however, does not appear to correlate with the IR performance – better translation does not necessarily yield better retrieval. We discuss in detail the contribution of the individual techniques and state-of-the-art features and provide future research directions

Document Meta-Information as Weak Supervision for Machine Translation

Data-driven machine translation has advanced considerably since the first pioneering work in the 1990s with recent systems claiming human parity on sentence translation for highresource tasks. However, performance degrades for low-resource domains with no available sentence-parallel training data. Machine translation systems also rarely incorporate the document context beyond the sentence level, ignoring knowledge which is essential for some situations. In this thesis, we aim to address the two issues mentioned above by examining ways to incorporate document-level meta-information into data-driven machine translation. Examples of document meta-information include document authorship and categorization information, as well as cross-lingual correspondences between documents, such as hyperlinks or citations between documents. As this meta-information is much more coarse-grained than reference translations, it constitutes a source of weak supervision for machine translation. We present four cumulatively conducted case studies where we devise and evaluate methods to exploit these sources of weak supervision both in low-resource scenarios where no task-appropriate supervision from parallel data exists, and in a full supervision scenario where weak supervision from document meta-information is used to supplement supervision from sentence-level reference translations. All case studies show improved translation quality when incorporating document meta-information