Abstract: An online system receives content items from publishing users for presentation to other users. When selecting content for presentation to users, the online system accounts for amounts of compensation from publishing users when presenting content items. To prevent publishing users from exploiting content selection by the online system to obtain disproportionate presentation of their content items relative to other publishing users, the online system generates an estimated amount of revenue from various publishing users from presenting their content items. The online system compares an amount of compensation received from a publishing user to the estimated amount of revenue from the publishing user, and generates clusters of content items from the publishing user for review if the amount of compensation is at least a threshold amount less than the estimated amount of revenue.