Tampering - What many Managers do because they do not understand Variation. Basically it is the unnecessary adjustments (decisions) made in an effort to compensate for common cause variation.
Common Cause Variation - Systemic Variation that is found in all processes (i.e., inputs, new conditions, etc...) that defines the amount of variation that is within every system. Very very difficult to identify one source to common cause variation (as opposed to special cause variation).
Special Cause Variation - Are specific factors that attack a system like a virus attacks a body, they can usually be tracked down to a single source and removed. The key is identifying data in a timely manner, removing and preventing this cause from happening again. Spacial Cause results in big spikes to a system or process and must be dealt with in a completely different manner than Common Cause Variation.
CLICK TO ENLARGE SLIDE COPQ/PONC (below)
Additional Information (Post) on Worker Variation can be found at http://qualityg.blogspot.com/2007/04/worker-variation-capability-in.html
Additional Information (Post) on Cost of Poor Quality can be found at http://qualityg.blogspot.com/2007/06/cost-of-poor-quality-example.html