L.P. Quality Progress , May 1990, pp. Knowing the difference between common causes and special causes of variation is vital to improving a process or system.
Common causes are inherent in the process, while special causes occur under certain circumstances.
A process is stable or in statistical control if it has only common causes of variation. What is stable is not necessarily the magnitude of the variation but its predictability. Deming and Shewhart indicate that removal of special causes leads to stability. Fundamental changes in common causes also can increase stability. However, constant adjustment of a stable process will increase variation.