1. A method for setting up an information retrieval system and retrieving text information, comprising the steps of: preprocessing text including word, noun phrase and stop word identification; constructing system terms including setting up a term list and global weights; setting up and normalizing document vectors of all collected documents; constructing an interior differential term-document matrix D I mxn 1 such that each column in said interior differential term-document matrix is an interior differential document vector; decomposing, using SVD algorithm, D I, such that D I &equals;USV T, then with a proper k 1, defining the D I,k 1 &equals;U k 1 S k 1 V k 1 T to approximate D I ; defining an interior document likelihood function, P(x&verbar;D I ); constructing an exterior differential term-document matrix D E mxn 1, such that each column in said exterior differential term-document matrix is an exterior differential document vector; decomposing, using SVD algorithm, D E, such that D E &equals;USV T, then with a proper value of k 2, defining the D E,k 2 &equals;U k 2 S k 2 V k 2 T to approximate D E ; defining an exterior document likelihood function, P(x&verbar;D E ); and defining a posteriori function 25 P &it; ( D I, x ) = P &it; ( x &VerticalSeparator; D I ) &it; P &it; ( D I ) P &it; ( x &VerticalSeparator; D I ) &it; P &it; ( D I ) + P &it; ( x &VerticalSeparator; D E ) &it; P &it; ( D E ), where P(D I ) is set to be an average number of recalls divided by the number of documents in the data base and P(D E ) is set to be 1-P(D I ).