were exposed to the concepts of SPC, control charts came into use in the organization and
proved effective in identifying assignable causes; that is, periods of time in which the error
rates produced by the system were higher than those that could be justified by chance cause
alone. It is its ability to differentiate between assignable and chance causes that makes the
control chart so indispensable. Management must react differently to an assignable cause than
it does to a chance or random cause. Assignable causes are due to phenomena external to the
system, and they must be tracked down and their root causes eliminated. Chance or random
causes are part of the system itself. They can only be reduced or eliminated by making
changes in how the system operates. This may mean changes in work methods and procedures,
improved levels of operator training, different types of equipment and facilities, or
improved input materials, all of which are the responsibility of management. In the planning
process, many of the common causes identified were related to the experience, training, and
supervision of the individual planners, as well as poor input information from design and
development engineering. These common causes were systematically removed from the