Common-cause variation:


variation attributable to a large number of small sources of variation. Added together they may result in a high level of variation in a process and/or a large amount of waste. The sum of these common causes determines the inherent variation of the process and its capability (as defined by control limits).

Special-cause variation:


variation that is not attributable to the process all the time. Special causes are special circumstances.

Stable process

Shewhart showed that a stable process is one that shows no special-cause variation. Although it’s behaviour may be random, it will perform predictably – you can expect any performance between the upper and lower control limits. This defines the process capability.