7. A computer-implemented method, comprising: under control of one or more computer systems configured with executable instructions, analyzing keywords of previous searches to detect fixed phrases suitable for inclusion in a search index, each fixed phrase including a first keyword and a second keyword; determining a first count of the first keyword, a second count of the second keyword, and a mutual count of the first keyword and the second keyword appearing simultaneously in the previous searches; weighting each individual count of the first count, the second count, and the mutual count based at least in part on age of a respective individual count with respect to the previous searches; determining a pointwise mutual information score using the weighted first count of the first keyword, the weighted second count of the second keyword, and the weighted mutual count of the first keyword and the second keyword; associating a respective suitable fixed phrase with the search index based at least in part on the determined pointwise mutual score being greater than a threshold score; in response to receiving a search request, determine a relevance score for each of at least a portion of the keywords, including the respective suitable fixed phrase, with respect to a collection of content; provide at least one search result for presentation, the at least one search result at least referencing content selected from the collection of content based at least in part on the at least one relevance score; detect interaction with a search result of the provided at least one result from a user; and update the keywords associated with the search result.
14. A computer-implemented method according to claim 7 , wherein the age is a difference between a time when the respective individual count appeared in the previous searches and a current time.