Abstract: A system for adjusting reserve price for impressions of non-guaranteed delivery (“NDG”) advertising auctions includes a processor configured to retrieve a reserve price set by a publisher for an impression that is fillable by eligible advertisements to be streamed to users in a display content stream; and to retrieve user engagement information for users that engage the eligible advertisements. A statistical analyzer applies a statistical function to the user engagement information of an identified advertisement of the eligible advertisements, to generate a user engagement statistic for the identified advertisement related to a user engagement level. A reserve price adjuster dynamically adjusts the reserve price for the identified advertisement responsive to a value of the user engagement statistic, where the adjusted reserve price for the identified advertisement is different than the reserve price for at least another of the eligible advertisements based on different user engagement levels for each.