Six Sigma and Beyond: Statistical Process Control, Volume IV

In addition to common variation, there may also be special variation in a process. This form of variation is not a designed part of the system. It results from an unexpected change in one or more of the components of the process system (i.e., machinery, methods , measuring system, personnel, environment). The differences that result from these sources are not part of a chance pattern or system and are called special. Special causes of variation can usually be traced to single factors. A process that operates with special causes of variation is out of control. No single population curve can represent this operation because it is changing. Figure 6.4 represents an out-of-control system.

Figure 6.4: A typical out-of-control system.

The curves that represent an out-of-control operation are:

• Changing

• Unstable

• Unpredictable

The curves may be different because of their shapes , spreads (range or standard deviation), or locations (mean or median). Typical assignable causes associated with special variation are things that are:

• Unnatural

• Disturbed

• Unstable

• Mixed

• Shifting

• Unpredictable

• Inconsistent

• Out of the ordinary

• Different

• Important

• Significant