26. The document rating calculation method according to claim 17 , further comprising a necessary item determining step of specifying a predetermined item in the document which does not influence the document rating of the document, and excluding the item from candidates whose document ratings are to be calculated.
26. The document rating calculation method according to claim 17 , further comprising a necessary item determining step of specifying a predetermined item in the document which does not influence the document rating of the document, and excluding the item from candidates whose document ratings are to be calculated. 27. The document rating calculation method according to claim 26 , wherein the necessary item determining step changes a bit corresponding to an item which does not influence a rating of a document and included in a bit array having bits assigned to all items included in the document, to a value indicating that the bit is not used in calculation of the document rating.