Word Clustering for Persian Statistical Parsing

Syntactically annotated data like a treebank are used for training the statistical parsers. One of the main aspects in developing statistical parsers is their sensitivity to the training data. Since data sparsity is the biggest challenge in data oriented analyses, parsers have a malperformance if they are trained with a small set of data, or when the genre of the training and the test data are not equal. In this paper, we propose a word-clustering approach using the Brown algorithm to overcome these problems. Using the proposed class-based model, a more coarser level of the lexicon is created compared to the words. In addition, we propose an extension to the clustering approach in which the POS tags of the words are also taken into the consideration while clustering the words. We prove that adding this information improves the performance of clustering specially for homographs. In usual word clusterings, homographs are treated equally; while the proposed extended model considers the homographs distinct and causes them to be assigned to different clusters. The experimental results show that the class-based approach outperforms the word-based parsing in general. Moreover, we show the superiority of the proposed extension of the class-based parsing to the model which only uses words for clustering.