A content delivery system adjusts bids across publishing channels to account for historical ratios that content was targeted to a content item's audience. A content campaign for users of a content delivery system is devised for two or more sponsored content providers. Targeting criteria for the content item is used to define an audience for the content item, and a sample group from that audience is chosen. The ratio of content impressions among the content providers is identified for the sample group among prior content presentations to these users. For the content item, based on the current ratio of presenting content across a secondary publishing channel and a benchmark publishing channel, a channel control factor is adjusted for the secondary publishing channel based on a numerical comparison of these two ratios. This adjusted channel control factor adjusts the bid price per impression for displaying content by the content campaign.
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