Displays and controls are elements in an interface between people and machines. Figure
33-1 illustrates the interface and elements involved for people and for machines. Machines
perform some functions and people control the operations of most machines. The opera-
tor must know what the machine is doing to know how to adjust the controls. Displays
provide information about the machine to the operator, and the operator must sense the
information provided by the display. The operator processes the information, decides what
action to take, acts on some control, and affects the machine operation. If there is a failure
in the person-machine loop, it may affect the performance of the system. Failures on the
person side are usually called errors.