11. A database search system comprising: a storage element storing keywords entered by at least one user for a plurality of search terms and information related to converted transactions associated with a plurality of searches, the plurality of searches having been performed on the keywords; and a processing system coupled to the storage element, the processing system operative to perform steps comprising: estimating a general transaction probability; estimating unigram keyword probabilities; determining conditional probability values associated with the keywords based on the converted transactions; estimating keyword transaction probabilities based on the general transaction probability, the unigram keyword probabilities, and the conditional probability values; and generating expected transaction values based on the keyword transaction probabilities, wherein the expected transaction values are generated according to: E ⁡ ( T ❘ k , S ) = P ⁡ ( T ❘ k , S ) * ∑ ∀ t ⁢ R ⁡ ( t ❘ k , S ) C ⁡ ( t ❘ k , S ) , where C(t|k,S) is a non-zero value.
11. A database search system comprising: a storage element storing keywords entered by at least one user for a plurality of search terms and information related to converted transactions associated with a plurality of searches, the plurality of searches having been performed on the keywords; and a processing system coupled to the storage element, the processing system operative to perform steps comprising: estimating a general transaction probability; estimating unigram keyword probabilities; determining conditional probability values associated with the keywords based on the converted transactions; estimating keyword transaction probabilities based on the general transaction probability, the unigram keyword probabilities, and the conditional probability values; and generating expected transaction values based on the keyword transaction probabilities, wherein the expected transaction values are generated according to: E ⁡ ( T ❘ k , S ) = P ⁡ ( T ❘ k , S ) * ∑ ∀ t ⁢ R ⁡ ( t ❘ k , S ) C ⁡ ( t ❘ k , S ) , where C(t|k,S) is a non-zero value. 12. The database search system of claim 11 , wherein the processing system is further operative to perform a step of periodically updating the conditional probability values.