37. The system of claim 36 , 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.
37. The system of claim 36 , 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. 38. The system of claim 37 , wherein a search result impression is a search result that is selected by a user or ranked above a search result that is selected by a user in a ranked list of search results provided for the query.