Structural Variation is of special note here. Some folks break out variation into four categories:
Special Cause
Common Cause
Tampering
Structural Variation
You can consider both Structural and Tampering variation as Common Cause variation. For example, the semi-conductor industry, as well as many other industries, have �clean rooms� or other climate and interference controlled (noise, vibration, temperature, humidity, air particle level, etc.) facilities. By not operating in air conditioning during the summer, management has built into the system a significant source of pollution / interference.
Structural Variation is regular, systematic changes in output. Typical examples include seasonal patterns and long-term trends.
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