Routine or common-cause variation. Even measurements from a stable process exhibit these random ups and downs. When process measurements exhibit only common-cause variation, the measurements stay within expected limits.
Abnormal or special-cause variation. Examples of special-cause variation include a change in the process mean, points above or below the control limits, or measurements that trend up or down. These changes can be caused by factors such as a broken tool or machine, equipment degradation, and changes to raw materials. A change or defect in the process is often identifiable by abnormal variation in the process measurements.
Control chart performance is dependent on the sampling scheme used. The sampling plan should be rational, that is, the subgroups are representative of the process. Rational subgrouping means that you sample from the process by selecting subgroups in such a way that special causes are more likely to occur between subgroups rather than within subgroups.

Help created on 9/19/2017