10. A method comprising: identifying a corpus of campaigns, each campaign including constraints, delivery goals and selection criteria for the delivery of one or more associated content items; determining, by one or more processors, a first campaign from the corpus of campaigns including evaluating constraints associated with delivery of the one or more content items for a first campaign and determining that the constraints include a budget constraint that is sufficient so as to enable the first campaign to be included in an experiment while still enabling the first campaign to meet its respective delivery goals; identifying a content item for presentation to users in response to requests for content, the content item including a link to a resource and being part of the first campaign associated with a content item sponsor, the campaign being associated with a geographic region, the geographic region including one or more geographic locations; selecting at least one of the one or more geographic locations; pausing presentation of the content item in the selected at least one of the one or more geographic locations during a first time period; presenting the content item in the selected at least one of the one or more geographic locations during a second different time period; and measuring content item effectiveness of the content item including comparing conversions associated with the resource during the first time period and conversions associated with the resource during the second different time period in the selected one or more geographic locations.
10. A method comprising: identifying a corpus of campaigns, each campaign including constraints, delivery goals and selection criteria for the delivery of one or more associated content items; determining, by one or more processors, a first campaign from the corpus of campaigns including evaluating constraints associated with delivery of the one or more content items for a first campaign and determining that the constraints include a budget constraint that is sufficient so as to enable the first campaign to be included in an experiment while still enabling the first campaign to meet its respective delivery goals; identifying a content item for presentation to users in response to requests for content, the content item including a link to a resource and being part of the first campaign associated with a content item sponsor, the campaign being associated with a geographic region, the geographic region including one or more geographic locations; selecting at least one of the one or more geographic locations; pausing presentation of the content item in the selected at least one of the one or more geographic locations during a first time period; presenting the content item in the selected at least one of the one or more geographic locations during a second different time period; and measuring content item effectiveness of the content item including comparing conversions associated with the resource during the first time period and conversions associated with the resource during the second different time period in the selected one or more geographic locations. 11. The method of claim 10 further comprising presenting an alternative content item during the first time period.