6. The method according to claim 5 , said steps further comprising: calculating weight values indicating strengths of combinations each term to obtain an m-by-n lexical mapping matrix T, which is defined as: 
 T=Q t P wherein t represents the transposition of a matrix, and each weight value of the lexical mapping matrix T (W(r,i)) is defined as: W ⁡ ( r , i ) = ∑ k = 1 S ⁢ [ Exp ⁡ ( k , i ) ( Etf ⁡ ( i ) * Ewf ⁡ ( k ) ) ⁢ Naive ⁡ ( k , r ) ( Ntf ⁡ ( r ) * Nwf ⁡ ( k ) ) ] ; and outputting the calculated weight values.
10. A computer readable storage medium comprising a set of instructions stored thereon for causing a computer to perform the method of claim 6 .