14. A non-transitory computer-readable medium encoded with a document ranking application comprising modules executable by a processor and configured to rank document data 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 a plurality of documents that each comprises 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 of the plurality of documents, the term frequency value comprising a total number of occurrences of the key term in a particular document; a negation module to: retrieve negation terms and negation rules from a memory, the negation terms comprising prefix negation terms and suffix negation terms, the negation rules comprising a prefix negation rule comprising that a prefix negation term appear before the key term in a same sentence as the key term within a selected proximity and a suffix negation rule comprising that a suffix negation term appear after the key term in the same sentence as the key term within the selected proximity, the selected proximity comprising a selected number of words; compare the negation terms to other terms in each document within the selected proximity of each occurrence of the key term and according to the negation rules; exclude a particular occurrence of the key term in each document if at least one negation term matches another term according to the negation rules within the selected proximity of the particular occurrence of the key term; and determine a corresponding term weight value for the key term in each document 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 based on the corresponding term frequency value and corresponding term weight value, the relevancy ranking value comprising a document relevance value comprising a ratio comprising the term weight value in a numerator of the ratio and the term frequency value in a denominator of the ratio; and a user interface module to generate a list of document data for display, the list identifying the plurality of documents in order based on the corresponding relevancy ranking value of each document.
14. A non-transitory computer-readable medium encoded with a document ranking application comprising modules executable by a processor and configured to rank document data 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 a plurality of documents that each comprises 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 of the plurality of documents, the term frequency value comprising a total number of occurrences of the key term in a particular document; a negation module to: retrieve negation terms and negation rules from a memory, the negation terms comprising prefix negation terms and suffix negation terms, the negation rules comprising a prefix negation rule comprising that a prefix negation term appear before the key term in a same sentence as the key term within a selected proximity and a suffix negation rule comprising that a suffix negation term appear after the key term in the same sentence as the key term within the selected proximity, the selected proximity comprising a selected number of words; compare the negation terms to other terms in each document within the selected proximity of each occurrence of the key term and according to the negation rules; exclude a particular occurrence of the key term in each document if at least one negation term matches another term according to the negation rules within the selected proximity of the particular occurrence of the key term; and determine a corresponding term weight value for the key term in each document 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 based on the corresponding term frequency value and corresponding term weight value, the relevancy ranking value comprising a document relevance value comprising a ratio comprising the term weight value in a numerator of the ratio and the term frequency value in a denominator of the ratio; and a user interface module to generate a list of document data for display, the list identifying the plurality of documents in order based on the corresponding relevancy ranking value of each document. 15. The non-transitory computer-readable medium of claim 14 wherein at least one negation rule comprises a suffix negation rule comprising that at least one suffix negation term appear after the key term in the same sentence without the prefix negating term within the selected proximity.