Methods, systems, and apparatus for receiving data corresponding to a contact by a user detected at a touch-sensitive device. Variations in the contact over time are analyzed, and an anchor confidence score that is indicative of whether the contact represents a user input made using the touch-sensitive device is determined based at least on the analysis of the variations in the contact over time. The contact is classified as an anchor based at least on the anchor confidence score. Based on classifying the contact as an anchor, the contact is not processed as a user input to the touch-sensitive device.