Distinguishing between the types of causes is critical because the appropriate managerial actions are quite different for each. Without this distinction, management will never be able to tell real improvement from mere adjustment of the process or tampering.
In practice, the most important difference to grasp first is the difference between special cause variation and common cause variation.
The strategy for special causes is simple: Get timely data. Investigate immediately when the data signals a special cause is/was present. Find out what was different or special about that point. Seek to prevent bad causes from recurring. Seek to keep good causes happening.
The strategy for improving a common cause system is more subtle. In a common cause situation, all the data are relevant, not just the most recent or offending figure. If you have data each month for the past two years, you will need to look at all of that data.
. 
