24. The method of claim 23 , wherein the first probability is a recent impression probability for the query for the recent time period, wherein the recent impression probability corresponds to a ratio of (i) a count of search result impressions selected by users in the recent time period to (ii) a count of all search result impressions presented to users in the recent time period.
24. The method of claim 23 , wherein the first probability is a recent impression probability for the query for the recent time period, wherein the recent impression probability corresponds to a ratio of (i) a count of search result impressions selected by users in the recent time period to (ii) a count of all search result impressions presented to users in the recent time period. 25. The method of claim 24 , wherein the second probability is an overall impression probability for the query for the overall time period, wherein the overall impression probability corresponds to a ratio of (i) a count of search result impressions selected by users in the overall time period to (ii) a count of all search result impressions presented to users in the overall time period.