Methods, systems, and apparatus, including computer programs encoded on a computer-readable storage medium, for reporting an externality effect. A method includes: identifying an auction associated with a content item request, the auction including one or more eligible inventory items for presentation responsive to the content item request and including a first inventory item associated with a first entity; evaluating results of the auction including determining an externality effect of another eligible inventory item in the auction on the first inventory item; and reporting the externality effect.
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