Efficient corpus selection for statistical machine translation

In our world of international communications, machine translation has become a key technology essential. Several pproaches exist, but in recent years the so-called Statistical Machine Translation (SMT) is considered the most promising. In this approach, knowledge is automatically extracted from examples of translations, called parallel texts, and monolingual data in the target language. Statistical machine translation is a data driven process. This is commonly put forward as a great advantage of statistical approaches since no human intervention is required, but this can also turn into a problem when the necessary development data are not available, are too small or the domain is not appropriate. The research presented in this thesis is an attempt to overcome barriers to massive deployment of statistical machine translation systems: the lack of parallel corpora. A parallel corpus is a collection of sentences in source and target languages that are aligned at the sentence level. Most existing parallel corpora were produced by professional translators. This is an expensive task in terms of money, human resources and time. This thesis provides methods to overcome this need by exploiting the easily available huge comparable and monolingual data collections. We present two effective architectures to achieve this.In the first part of this thesis, we worked on the use of comparable corpora to improve statistical machine translation systems. A comparable corpus is a collection of texts in multiple languages, collected independently, but often containing parts that are mutual translations. The size and quality of parallel contents may vary considerably from one comparable corpus to another, depending on various factors, including the method of construction of the corpus. In any case, itis not easy to automatically identify the parallel parts. As part of this thesis, we developed an approach which is entirely based on freely available tools. The main idea of our approach is the use of a statistical machine translation system to translate all sentences in the source language comparable corpus to the target language. Each of these translations is then used as query to identify potentially parallel sentences from the target language comparable corpus. This research is carried out using an information retrieval toolkit. In the second step, the retrieved sentences are compared to the automatic translation to determine whether they are parallel to the corresponding sentence in source language. Several criteria wereevaluated such as word error rate or the translation edit rate (TER) and TERp. We conducted a very detailed experimental analysis to demonstrate the interest of our approach. We worked on comparable corpora from the news domain, more specifically, multilingual news agencies such as, "Agence France Press (AFP)", "Associate Press" or "Xinua News. " These agencies publish daily news in several languages. We were able to extract parallel texts from large collections of over three hundred million words for French-English and Arabic-English language pairs. These parallel texts have significantly improved our statistical translation systems. We also present a theoretical comparison of the model developed in this thesis with another approach presented in the literature. Various extensions are also discussed: automatic extraction of unknown words and the creation of a dictionary, detection and suppression of extra information, etc. In the second part of this thesis, we examined the possibility of using monolingual data to improve the translation model of a statistical system. The idea here is to replace parallel data by monolingual source or target language data. This research is thus placed in the context of unsupervised learning, since missing translations are produced by an automatic translation system, and after various filtering, reinjected into the system.