Shewhart has invented the tool called process behavior chart or control chart to provide guidance for improvement.
Control chart helps us to understand the difference between common and special causes.
A common-cause variation is a routine variation. If the data’s are within the limits, the process displays nothing except “routine variation”. Shewhart called a process that features only common-cause variation as being in statistical control The process is stable and predictable.
Special-cause variation always arrives as a surprise. It is the signal within a system. For example, when a points fall outside the limits of a process behavior chart , it’s a signal of exceptional variation. So you try to identify the causes of the exceptional variation.
We’ve all learned that the first mistake in interpreting data is to interpret noise as it were a signal, the second mistake in interpreting data is to fail to detect a signal when it is present.
But most often when we have only common-causes, we look at the process and we think it’s working well. So, things are maintaining as they are. By maintaining things as they are, we don’t improve. Common-causes doesn’t mean that we’re doing the right things right.
The discovery of two types of variation is the first step toward the Deming Management Theory, but most often we use control chart for monitoring purpose not for improvement purpose. If we are just using the control chart to monitor our process, we’re forgetting the major objectives of the tool and the philosophy behind.
As expressed by Wheeler, in his book understanding variation, the question is not whether or not the techniques will work – but rather whether or not you will make them work. Those who don’t use process behavior charts have no advantages over those who can’t.
Source : Wheeler, understanding variation, the key to Managing chaos…