50. The system of claim 48 wherein: the negation module is further configured to determine a number of negated occurrences for the key term in each document, the number of negated occurrences corresponding to a sum of the particular occurrences of the key term in each document where the at least one of the negation terms matches the other terms within the selected proximity of the particular occurrence of the key term; the term weight (TW) comprises a difference between the term frequency (TF) and the number of negated occurrences; and the ranking module is configured to determine the document relevance (DR) value comprising: DR = TW 3 TF + 1.
48. A ranking system comprising: a memory comprising at least one negation term and at least one negation rule; a processor comprising a document ranking application comprising modules executable by the processor to rank documents retrieved from a data source in response to a search request, the document ranking application comprising: a term frequency module to: query the data source to identify at least two documents, each document comprising a key term, the key term matching a search term in the search request; and determine a corresponding term frequency value for the key term in each document of the at least two documents, the term frequency value comprising a total number of occurrences of the key term in each document of the at least two documents; a negation module to: retrieve the at least one negation term and the at least one negation rule from the memory; compare the at least one negation term according to the at least one negation rule to other terms in each document of the at least two documents that are within a selected proximity of each occurrence of the key term to determine if each occurrence of the key term has a negative context; determine that the at least one negation term matches another term within the selected proximity of the particular occurrence of the key term according to the at least one negation rule and exclude a particular occurrence of the key term in at least one document for having the negative context; and determine a corresponding term weight value for the key term in each document of the at least two documents based on each occurrence of the key term that has not been excluded; a ranking module to determine a corresponding relevancy ranking value for each document of the at least two documents based on the corresponding term frequency value and corresponding term weight value; and a user interface module to generate a list of document data for display, the list identifying each document of the at least two documents in an order based on the relevancy ranking value of each document of the at least two documents.