Here the weight reflects the importance of the terms appearing in the entire document set. For every document in the document set (N), the weight for each index term needs to be calculated. Every document contains a document vector which has the weight for every index term that appears in that document. For example, if a word appears in all documents it will not have any value across the document’s set. For calculating term weights, a method of combining the importance of a word in a specific document in the whole document set is required. After we obtain the required values, the weights can be calculated .