Building Morphological Chains for Agglutinative Languages

In this paper, we build morphological chains for agglutinative languages by using a log-linear model for the morphological segmentation task. The model is based on the unsupervised morphological segmentation system called MorphoChains. We extend MorphoChains log linear model by expanding the candidate space recursively to cover more split points for agglutinative languages such as Turkish, whereas in the original model candidates are generated by considering only binary segmentation of each word. The results show that we improve the state-of-art Turkish scores by 12% having a F-measure of 72% and we improve the English scores by 3% having a F-measure of 74%. Eventually, the system outperforms both MorphoChains and other well-known unsupervised morphological segmentation systems. The results indicate that candidate generation plays an important role in such an unsupervised log-linear model that is learned using contrastive estimation with negative samples.Comment: 10 pages, accepted and presented at the CICLing 2017 (18th International Conference on Intelligent Text Processing and Computational Linguistics

Comparing rule-based and data-driven approaches to Spanish-to-Basque machine translation

In this paper, we compare the rule-based and data-driven approaches in the context of Spanish-to-Basque Machine Translation. The rule-based system we consider has been developed specifically for Spanish-to-Basque machine translation, and is tuned to this language pair. On the contrary, the data-driven system we use is generic, and has not been specifically designed to deal with Basque. Spanish-to-Basque Machine Translation is a challenge for data-driven approaches for at least two reasons. First, there is lack of bilingual data on which a data-driven MT system can be trained. Second, Basque is a morphologically-rich agglutinative language and translating to Basque requires a huge generation of morphological information, a difficult task for a generic system not specifically tuned to Basque. We present the results of a series of experiments, obtained on two different corpora, one being “in-domain” and the other one “out-of-domain” with respect to the data-driven system. We show that n-gram based automatic evaluation and edit-distance-based human evaluation yield two different sets of results. According to BLEU, the data-driven system outperforms the rule-based system on the in-domain data, while according to the human evaluation, the rule-based approach achieves higher scores for both corpora

A Trie-Structured Bayesian Model for Unsupervised Morphological Segmentation

In this paper, we introduce a trie-structured Bayesian model for unsupervised morphological segmentation. We adopt prior information from different sources in the model. We use neural word embeddings to discover words that are morphologically derived from each other and thereby that are semantically similar. We use letter successor variety counts obtained from tries that are built by neural word embeddings. Our results show that using different information sources such as neural word embeddings and letter successor variety as prior information improves morphological segmentation in a Bayesian model. Our model outperforms other unsupervised morphological segmentation models on Turkish and gives promising results on English and German for scarce resources.Comment: 12 pages, accepted and presented at the CICLING 2017 - 18th International Conference on Intelligent Text Processing and Computational Linguistic

Identificación de cláusulas y chunks para el Euskera, usando Filtrado y Ranking con el Perceptron

Este artículo presenta sistemas de identificación de chunks y cláusulas para el euskera, combinando gramáticas basadas en reglas con técnicas de aprendizaje automático. Más concretamente, se utiliza el modelo de Filtrado y Ranking con el Perceptron (Carreras, Màrquez y Castro, 2005): un modelo de aprendizaje que permite identificar estructuras sintácticas parciales en la oración, con resultados óptimos para estas tareas en inglés. Este modelo permite incorporar nuevos atributos, y posibilita así el uso de información de diferentes fuentes. De esta manera, hemos añadido información lingüística en los algoritmos de aprendizaje. Así, los resultados del identificador de chunks han mejorado considerablemente y se ha compensado la influencia del relativamente pequeño corpus de entrenamiento que disponemos para el euskera. En cuanto a la identificación de cláusulas, los primeros resultados no son demasiado buenos, debido probablemente al orden libre del euskera y al pequeño corpus del que disponemos actualmente.This paper presents systems for syntactic chunking and clause identification for Basque, combining rule-based grammars with machine-learning techniques. Precisely, we used Filtering-Ranking with Perceptrons (Carreras, Màrquez and Castro, 2005): a learning model that recognizes partial syntactic structures in sentences, obtaining state-of-the-art performance for these tasks in English. This model allows incorporating a rich set of features to represent syntactic phrases, making possible to use information from different sources. We used this property in order to include more linguistic features in the learning model and the results obtained in chunking have been improved greatly. This way, we have made up for the relatively small training data available for Basque to learn a chunking model. In the case of clause identification, our preliminary results are low, which suggest that this is due to the free order of Basque and to the small corpus available.Research partly funded by the Basque Government (Department of Education, University and Research, IT-397-07), the Spanish Ministry of Education and Science (TIN2007-63173) and the ETORTEK-ANHITZ project from the Basque Government (Department of Culture and Industry, IE06- 185)

Unsupervised joint PoS tagging and stemming for agglutinative languages

This is an accepted manuscript of an article published by Association for Computing Machinery (ACM) in ACM Transactions on Asian and Low-Resource Language Information Processing on 25/01/2019, available online: https://doi.org/10.1145/3292398 The accepted version of the publication may differ from the final published version.The number of possible word forms is theoretically infinite in agglutinative languages. This brings up the out-of-vocabulary (OOV) issue for part-of-speech (PoS) tagging in agglutinative languages. Since inflectional morphology does not change the PoS tag of a word, we propose to learn stems along with PoS tags simultaneously. Therefore, we aim to overcome the sparsity problem by reducing word forms into their stems. We adopt a Bayesian model that is fully unsupervised. We build a Hidden Markov Model for PoS tagging where the stems are emitted through hidden states. Several versions of the model are introduced in order to observe the effects of different dependencies throughout the corpus, such as the dependency between stems and PoS tags or between PoS tags and affixes. Additionally, we use neural word embeddings to estimate the semantic similarity between the word form and stem. We use the semantic similarity as prior information to discover the actual stem of a word since inflection does not change the meaning of a word. We compare our models with other unsupervised stemming and PoS tagging models on Turkish, Hungarian, Finnish, Basque, and English. The results show that a joint model for PoS tagging and stemming improves on an independent PoS tagger and stemmer in agglutinative languages.This research is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) with the project number EEEAG-115E464.Published versio