Choosing the Optimal Segmentation Level for POS Tagging of the Quranic Arabic

As a morphologically rich language, Arabic poses special challenges to Part-of-Speech (POS) tagging. Words in Arabic texts often contain several segments; each has its own POS category. The choice of the segmentation level or the input unit, word-based or morpheme-based, is a major issue in designing any Arabic natural language processing system. In word-based approaches, words are used the atomic units of the language. In this case, composite POS tags are assigned to words. Therefore, large amounts of training data are required in order to ensure statistical significance. They suffer from the problems of data sparseness and unknown words. In case of morpheme-based approaches, morpheme components of words are used as the atomic units. This, however, results in high level of ambiguity rate and also small size of context for resolving such ambiguity because the span of the n-gram might be limited to a single word. This paper compares and contrasts the morpheme-based and word-based statistical POS tagging strategies. This paper evaluates the tagging performance of three statistical models, namely, the Arabic HMM POS tagger with the prefix guessing models, the Arabic HMM POS tagger with the linear interpolation guessing models and the TnT tagger, given training data from both morpheme-based and word-based tokenization levels. It also studies the influence of each choice on the tagging performance of the Arabic POS tagging models, in terms of the tagging accuracy and the time complexity. In addition, this paper also evaluates the tagging performance of several stochastic models, given training data from both segmentation levels. Results show that the morpheme-based POS tagging strategy is more adequate for the purpose of training statistical POS tagging models as it provides a better overall tagging accuracy and a much faster training and tagging time.