4. The method of claim 3 , wherein projecting the third document-word matrix onto a lower dimensional space comprises: decomposing the third document-word matrix into a product of at east three matrices; and applying dimensional reduction to the product of the at least three matrices and defining a transformed document-word matrix.
6. The method of claim 4 , wherein decomposing and applying dimensional reduction comprises: using an incremental singular value decomposition technique by iteratively applying latent semantic analysis to submatrices of the third document-word matrix.