Structural Variation is of special note here. Some folks break out variation into four categories:
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.