sentences:  One method for allowing rarer terms to be included and common words to be excluded by automated techniques would be a relative frequency approach where frequency of a word in a document is compared to frequency in the database as a whole.Therefore, a term that occurs more often in a document than might be expected based on the rest of the database could then be used as an index term, and terms that occur equally frequently throughout will be excluded.Another problem with automated extraction is that it does not recognise when a concept is discussed but is not identified in the text by an indexable keyword.