Every system has variation. Common cause variation is the variation due to the current system. Dr. Deming increased his estimate of variation due to the system (common cause variation) to 97% (earlier in his life he cited figures around 80%). Special cause variation is that due to some special (not part of the system) cause.

The control chart (in addition to other things) helps managers to avoid tampering (taking action on common cause variation as though it were a special cause). In order to take action against the results of common cause variation the performance of the system the system itself must be changed. A systemic improvement approach is needed.

To take action against a special cause, that isolated special cause can be examined. Unfortunately that approach (the one we tend to use almost all the time) is the wrong approach for systemic problems (which Deming estimated at 97% of the problems).

That doesn’t mean it is not possible to improve results by treating all problems as some special event. Examining each failure in isolation is just is not as effective. Instead examine the system that produced those results is the best method. The control chart provides a measurement of the system. The chart will show what the process is capable of producing and how much variation is in the system now.

If you would like to reduce the variation picking the highest data values (within the control limits) and trying to study them to figure out why they are so high is not effective. Instead you should study the whole system and figure out what systemic changes to make. One method to encourage this type of thinking is asking why 5 times. It seeks to find the systemic reasons for individual results.

Related: SPC - History - Understanding Variation by Tom Nolan and Lloyd Provost (highly recommended) - Deming on Management - Responding to Variation: Common Causes