8. The method of claim 7 , further comprising: determining a user state for a user for which the content item request was received, the user state specifying at least one search query that was previously submitted by the user; and determining, based on the at least one search query specified in the user state and at least the carry-over effect included in the probability distribution associated with the at least one search query, the likelihood of the specified outcome occurring following presentation of an advertisement that is targeted using the keyword in the final set of keywords; and generating a corresponding bid amount for the keyword based the likelihood of the specified outcome determined from at least the carry-over effect.
7. The method of claim 1 , further comprising: receiving a content item request that includes data that match a keyword in the final set of keywords.