1. A method performed by data processing apparatus, the method comprising: receiving, from a user device and over a data communication network, a content item request; identifying, in response to the content item request, eligible content items that are eligible for presentation in response to the content item request, the eligible content items including a given content item having a given characteristic; identifying, for the given content item, a bid value specifying an amount a content item provider is willing to pay for user interaction with the given content item; identifying, for the given content item, a predicted performance measure specifying a predicted performance for the given content item; identifying a first set of content items that have the given characteristic, the first set of content items including the given content item; determining, for the given content item, a weighting factor that is based on a variance in at least one performance measure for the content items in the first set of content items, the weighting factor for the given content item being different than a weighting factor for another eligible content item that is included in a second set of content items different from the first set of content items, the second set of content items including content items that do not have the given characteristic; adjusting, using weighting factor for the given content item, the predicted performance measure to generate an adjusted predicted performance measure; determining a rank score for the given content item using the bid value for the given content item and the adjusted predicted performance measure for the given content item; and providing, to the user device and based on the rank score for the given content item, data that initiates presentation of the given content item at the user device.