A disclosed example method to determine audience duplication in cross-media campaigns involves determining a first duplicated audience reach between a total television audience and an internet publisher audience for a demographic group. The total television audience is representative of audience members across a plurality of television networks, and the internet publisher audience is representative of audience members across one of a plurality of internet publishers. The example method involves determining a second duplicated audience reach between the total television audience and the internet publisher audience for a media campaign based on the first duplicated audience reach. The example method involves determining a third duplicated audience reach between a television network audience and the internet publisher audience for the media campaign based on the second duplicated audience reach. The television network audience is representative of audience members exposed to the media campaign via one of the plurality of television networks.