Methods, systems, and apparatus include computer programs encoded on a computer-readable storage medium for measuring lift. A method includes: receiving requests for content for which a first content item is an eligible content item; delivering the first content item along with a first pixel that identifies the delivery of the first content item to a first portion of the received requests; delivering a second, different content item along with a second pixel that identifies the non-delivery of the first content item to a second portion of the received plurality of requests rather than the first content item; evaluating searches performed by users that received the first content item as compared to users that received the second content item including using the first pixel and second pixel in evaluating search logs associated with search requests performed by users; and presenting lift data to a sponsor associated with the first content item.