S D =??S T *TF TD, ST has the value set forth above, TFTD=Robertson's term frequency for Term T in Document D =NTD/(NTD+K1+K2*(LD/L0)), where: NTD is a number of times the term T occurs in document D, LD is a length of document D, L0 is an average length of a document in the subset of documents indexed, and K1 and K2 are constants. 21.