1. A method performed by data processing apparatus, the method comprising: identifying a primary ranking signal and a set of auxiliary ranking signals for ranking a set of criteria for a content item, wherein the primary ranking signal defines a first attribute for one of the criteria in the set of criteria and each auxiliary ranking signal in the set of auxiliary ranking signals a) defines a second attribute for one of the criteria in the set of criteria and b) is different than the primary ranking signal; for each particular criterion in the set of criteria: identifying a primary score representing a value of the primary ranking signal for the particular criterion; identifying a set of auxiliary scores for the particular criterion, each auxiliary score representing a value of an auxiliary ranking signal of the set of auxiliary ranking signals for the particular criterion; adjusting each auxiliary score in the set of auxiliary scores to generate adjusted auxiliary scores, the adjusting comprising applying, to at least a portion of the auxiliary scores, a transformation function that reduces an amount of skewness among the auxiliary scores that are associated with a particular auxiliary ranking signal from the set of auxiliary ranking signals; and determining a ranking score for the particular criterion based on a function of the primary score for the particular criterion and the adjusted auxiliary scores, the function suppressing effects of adjusted auxiliary scores for the particular criterion that do not satisfy a specified value and boosting the ranking score using adjusted auxiliary scores for the particular criterion that satisfy the specified value; selecting one or more criteria in the set of criteria for which to associate the content item based on the ranking score for each particular criterion; and providing the content item in response to receiving data specifying one or more of the selected criteria.
1. A method performed by data processing apparatus, the method comprising: identifying a primary ranking signal and a set of auxiliary ranking signals for ranking a set of criteria for a content item, wherein the primary ranking signal defines a first attribute for one of the criteria in the set of criteria and each auxiliary ranking signal in the set of auxiliary ranking signals a) defines a second attribute for one of the criteria in the set of criteria and b) is different than the primary ranking signal; for each particular criterion in the set of criteria: identifying a primary score representing a value of the primary ranking signal for the particular criterion; identifying a set of auxiliary scores for the particular criterion, each auxiliary score representing a value of an auxiliary ranking signal of the set of auxiliary ranking signals for the particular criterion; adjusting each auxiliary score in the set of auxiliary scores to generate adjusted auxiliary scores, the adjusting comprising applying, to at least a portion of the auxiliary scores, a transformation function that reduces an amount of skewness among the auxiliary scores that are associated with a particular auxiliary ranking signal from the set of auxiliary ranking signals; and determining a ranking score for the particular criterion based on a function of the primary score for the particular criterion and the adjusted auxiliary scores, the function suppressing effects of adjusted auxiliary scores for the particular criterion that do not satisfy a specified value and boosting the ranking score using adjusted auxiliary scores for the particular criterion that satisfy the specified value; selecting one or more criteria in the set of criteria for which to associate the content item based on the ranking score for each particular criterion; and providing the content item in response to receiving data specifying one or more of the selected criteria. 7. The method of claim 1 , further comprising normalizing auxiliary scores for each auxiliary ranking signal.