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. |
