Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving a notification of an ad space, the ad space being for presentation in a user interface of an application executing on a client device, sending, to the client device, instructions configured to be executed by the client device for determining view-ability data of the ad space, the view-ability data comprising whether the ad space is within a viewport of the user interface, receiving, from the client device, the view-ability data as determined by the instructions at a first time instance, calculating a predicted view-ability that the ad space is positioned within the viewport at a second time instance after the first time instance, and sending, to a plurality of bidders, a bid request for bidding on an auction of the ad space, the bid request comprising the predicted view-ability.