External-signal influence on content item performance is determined. Content item performance data is received that reflects historic performance of a content item for multiple presentations of the content item. Signal data is received that corresponds to at least one signal that is temporally correlated with the content item performance data and that is external to each user, publisher and content provider involved in any of the presentations. Using the content item performance data and the signal data, an influence value for the signal with regard to the content item is determined. A content item prediction model is modified based on the influence value.