LRLW-LSI: an improved latent semantic indexing (LSI) text classifier

The task of Text Classification (TC) is to automatically assign natural language texts with thematic categories from a predefined category set. And Latent Semantic Indexing (LSI) is a well known technique in Information Retrieval, especially in dealing with polysemy (one word can have different meanings) and synonymy (different words are used to describe the same concept), but it is not an optimal representation for text classification. It always drops the text classification performance when being applied to the whole training set (global LSI) because this completely unsupervised method ignores class discrimination while only concentrating on representation. Some local LSI methods have been proposed to improve the classification by utilizing class discrimination information. However, their performance improvements over original term vectors are still very limited. In this paper, we propose a new local Latent Semantic Indexing method called "Local Relevancy Ladder-Weighted LSI" to improve text classification. And separate matrix singular value decomposition (SVD) was used to reduce the dimension of the vector space on the transformed local region of each class. Experimental results show that our method is much better than global LSI and traditional local LSI methods on classification within a much smaller LSI dimension.