13. A method comprising: providing a corpus of campaigns, each campaign associated with at least one content item, the at least one content item including a link to a resource and being associated with a sponsor of the content item, where a campaign defines parameters for presentation that specify circumstances under which the at least one content item is presented to users in response to received content item requests and constraints and delivery goals for the at least one content item; filtering, by one or more processors, the corpus by identifying one or more candidate campaigns in the corpus of campaigns, the filtering including evaluating constraints associated with delivery of the at least one content item for a given campaign and determining that the constraints include a budget constraint that is sufficient so as to enable the given campaign to be included in an experiment while still enabling the given campaign to meet its respective delivery goals, wherein each of the identified one or more candidate campaigns is associated with a geographic region and includes one or more constraints that permit pausing distribution of the content item in the campaign in a portion of the geographic region without compromising delivery goals; and for each of the identified one or more candidate campaigns, determining one or more geographic locations that are included in the geographic region, the determined one or more geographic locations being substantially representative of the geographic region; pausing presentation of the at least one content item in the determined one or more geographic locations for a time period; and measuring content item effectiveness of the at least one content item during the time period, wherein measuring content item effectiveness includes comparing a measure of effectiveness of the at least one content item during the time period for one or more paused and one or more unpaused geographic locations.
13. A method comprising: providing a corpus of campaigns, each campaign associated with at least one content item, the at least one content item including a link to a resource and being associated with a sponsor of the content item, where a campaign defines parameters for presentation that specify circumstances under which the at least one content item is presented to users in response to received content item requests and constraints and delivery goals for the at least one content item; filtering, by one or more processors, the corpus by identifying one or more candidate campaigns in the corpus of campaigns, the filtering including evaluating constraints associated with delivery of the at least one content item for a given campaign and determining that the constraints include a budget constraint that is sufficient so as to enable the given campaign to be included in an experiment while still enabling the given campaign to meet its respective delivery goals, wherein each of the identified one or more candidate campaigns is associated with a geographic region and includes one or more constraints that permit pausing distribution of the content item in the campaign in a portion of the geographic region without compromising delivery goals; and for each of the identified one or more candidate campaigns, determining one or more geographic locations that are included in the geographic region, the determined one or more geographic locations being substantially representative of the geographic region; pausing presentation of the at least one content item in the determined one or more geographic locations for a time period; and measuring content item effectiveness of the at least one content item during the time period, wherein measuring content item effectiveness includes comparing a measure of effectiveness of the at least one content item during the time period for one or more paused and one or more unpaused geographic locations. 14. The method of claim 13 where filtering the corpus further comprises filtering out campaigns that do not have a relatively uniform distribution over the geographic region.