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.
6. The method of claim 1 , further comprising: determining, for each particular auxiliary ranking signal, a maximum value of auxiliary scores for the auxiliary ranking signal, the maximum value being based on values of the auxiliary scores for the auxiliary ranking signal across the set of criteria and being less than a highest auxiliary value for the auxiliary ranking signal across the set of criteria; identifying a particular auxiliary score for the auxiliary ranking signal that represents a value for the auxiliary ranking signal and that has a value that exceeds the maximum value; and reducing the particular auxiliary score to a value that is equal to or less than the maximum value.