I have preached this one before but it is never wrong to give this message to quality ears-Know the difference between common cause and special cause variation. Don't just understand it in the context of an SPC chart. It is more than that. This is a way of thinking. It will govern the way you see the world. It will help you as a spouse, parent, and employee.
Overeaction! Unecessary adjustments. Sound familiar? Common cause variation is completely random in nature. If a process is in control, it is being governed by common cause variation. A plotted point has a 50% probability of falling above the center line and a 50% chance of falling below the centerline. It is analogous to flipping an honest coin to get heads (above the center line) or tails (below the centerline).
Special cause variation is not random. Something unique happened to cause the special occurrence. The random pattern of behavior was broken by a special event.
If you can get the people who run your machines and test your products to understand this concept, I promise it will change your business. How do I know? I lived it! It took me years to really understand it but once I did, I wanted others to share my knowledge.
Think about the way you are as a parent. Is your teenager behaving badly because of hormones (common cause variation for eight graders) or because her friends were mean to her at school (special cause)?
At work, do you have machines that are easily adjustable? How does an operator know when to "turn the knob"? The knob should not be turned when the process is being driven by a special cause. Don't touch the knob! Find the special cause. Turning the knob unecessarily will add variation to the process.