The computer-based control system of an oil refinery logs all the inappropriate commands made by its operators (i.e., commands that are ambiguous or uninterpretable). This system is used to monitor the way that the performance of the operators varies over the course of an eight-hour shift. The performance of 10 operators was monitored over many weeks until each of these had worked 20 shifts. For each operator the total number of errors made in each hour of the shift was computed.