1. A method for generating a reduced rank approximation for information retrieval, comprising: forming on a computer a term-by-document matrix A, wherein the elements of the matrix A represent a plurality of terms within a plurality of documents, the documents related to a plurality of topics; estimating via the computer a plurality of singular values corresponding to at least one of the topics; identifying via the computer a plurality of actual singular values each having a corresponding singular vector associated with the matrix A; selecting via the computer a subset of the actual singular values based on actual singular values that correspond to at least one of the estimated singular values; and determining via the computer a set of singular vectors based on the selected singular values, wherein the singular vectors provide an index for use during information retrieval.