4. The method of claim 3 wherein the critical phrases are determined by the formula: 
 ( C n-gram ) i,j =C i,j *idf Ci where C i,j relates to an appearance of ith phrase in a jth requirement document, and idf Ci is a weighting factor.
4. The method of claim 3 wherein the critical phrases are determined by the formula: 
 ( C n-gram ) i,j =C i,j *idf Ci where C i,j relates to an appearance of ith phrase in a jth requirement document, and idf Ci is a weighting factor. 6. The method of claim 4 wherein idf Ci is represented by the following formula: idf Ci = log ⁢  V   { v ⁢ : ⁢ ⁢ C i ∈ v }  where |V| is a total number of requirement documents in a corpus, and {v: C i εv} is number of requirement documents with the records of C i .