22. A computer-readable storage medium having processor executable instructions to process a search query having a plurality of search terms for searching for documents, by performing the acts of: segmenting the query to identify two or more units; expanding the query by selecting one or more substitutable units for at least one unit in the query; calculating a substitution probability for each substitutable unit; for each substitutable unit, calculating a co-occurrence probability with each of the remaining units in the search query; determining an occurrence probability for each substitutable unit; calculating a score based on the combination of the substitution probability, the co-occurrence probability, and occurrence probability; and ranking the documents in an order determined by the score.
22. A computer-readable storage medium having processor executable instructions to process a search query having a plurality of search terms for searching for documents, by performing the acts of: segmenting the query to identify two or more units; expanding the query by selecting one or more substitutable units for at least one unit in the query; calculating a substitution probability for each substitutable unit; for each substitutable unit, calculating a co-occurrence probability with each of the remaining units in the search query; determining an occurrence probability for each substitutable unit; calculating a score based on the combination of the substitution probability, the co-occurrence probability, and occurrence probability; and ranking the documents in an order determined by the score. 23. The computer-readable storage medium of claim 22 , further comprising processor executable instructions to cause a processor to perform the acts of calculating the substitution probability associated with a substitutable unit based on an occurrence frequency of the substitutable unit relative to the corresponding unit in related documents.