A system may select and serve content items based on a view probability. The view probability may be determined using a view probability predictive model and one or more current viewability variables. Based on the determined view probability output by the view probability predictive model, a content item may be selected and served. A script may be included with or otherwise associated with the data output to effect presentation of the selected content item. The script outputs data indicative of whether the served content item is viewed on a display of a device. The script may output data indicating that the served content item is viewed when at least a predetermined percentage of the pixels of the served content item are within a viewable region for a period of time. A served content item may be determined to not have been viewed if a predetermined period of time elapses.