3. The method of claim 2 , wherein identifying a second controlling keyword based on the tiebreaker rule comprises: determining, for each keyword among the two keywords, a selection score for the keyword based on a quality score for the keyword and a bid for the keyword; and identifying the second controlling keyword based on the selection scores for the two keywords.
3. The method of claim 2 , wherein identifying a second controlling keyword based on the tiebreaker rule comprises: determining, for each keyword among the two keywords, a selection score for the keyword based on a quality score for the keyword and a bid for the keyword; and identifying the second controlling keyword based on the selection scores for the two keywords. 4. The method of claim 3 , wherein determining a selection score for a keyword comprises determining a product of a quality score associated with a content item distributed based on the keyword and a bid associated with the keyword.