Is due to a factor that has "slipped" into the process causing unstable or unpredictable variation.
Are unpredictable variations that are abnormal to the process including human error, equipment failure, defective/changed raw materials, acid spills, power failures, etc.; failure to remove them can result is corrosion, scale, metal fatigue, lower equipment efficiency, increased maintenance costs, unsafe working conditions, wasted chemicals, increased down-time (plant shut-down...), etc.
Removal of all special causes of variation yields a process that is in statistical control.
Correctable by local personnel.
Common Causes of Variation
Account for 85-95% of quality problems
Are due to the many small sources of variation "engineered" into the process of the "system�.
Are naturally caused and are always present in the process because they are linked to the system's base ability to perform; it is the predictable and stable inherent variability resulting from the process operating as it was designed.
Standard deviation, s, is used as a measure of the inherent process variability; it helps describe the well-known normal distribution curve.
Correctable only by management; typically requires the repair/replacement of a system's component, or the system itself.
. 
