The first element is the "characteristic" or condition of the operating system to be measured. Specific characteristics are selected because a correlation exists between them and the system's performance. A characteristic can be the output of the system during any stage of processing (e.g. the heat energy produced by a furnace), or it may be a condition that is the result of the system (e.g. the temperature in the room which has changed because of the heat generated by the furnace). In an elementary school system, the hours a teacher works or the gain in knowledge demonstrated by the students on a national examination are examples of characteristics that may be selected for measurement, or control.