1. A method performed by data processing apparatus, the method comprising: identifying, for a content item, a bid value specifying an amount a content item provider is willing to pay for user interaction with the content item; identifying, for the content item, a predicted performance measure specifying a predicted performance for the content item; adjusting the predicted performance measure to generate an adjusted predicted performance measure, the adjusting being based on a weighting factor for the content item, the weighting factor for the content item being indicative of a confidence that the predicted performance measure will match an actual performance measure for the content item, the weighting factor for the content item being different than a weighting factor for another content item identified for inclusion in a ranking with the content item; determining a rank score for the content item using the bid value for the content item and the adjusted predicted performance measure for the content item; and providing the content item in response to a content item request based on the rank score for the content item.