Common Cause

All manufacturing and business processes contain many sources of variation. The differences may be so small as to be difficult to detect, but they are there. To successfully reduce variation, its sources must first be understood (common cause vs special cause). Less variation, less waste, more profits.

Common Cause vs Special Cause

Common cause variation is a constant system of chance. The sources of common cause variation are many in number but small in size. A process characterized by common cause variation is in a state of statistical control: its output is stable and predictable. The magnitude of common cause variation can be measured using control charts. The sources of common cause variation can be identified and quantified using Design of Experiments, regression analysis, and other statistical methods. The reduction of common cause variation requires actions on the system.  Example:  the total of two die.

Special cause variation affects processes in disruptive and unpredictable ways. The sources of special cause variation are relatively few in number but are large in size. A process driven by special cause variation is neither stable nor predictable. Special causes can be detected using control charts through out-of-control signals. The elimination of special causes requires local action on the process. Although special causes account for less than 20% of total variation in most processes, they must be removed before common cause variation can effectively be reduced.  Likely example:  a machine breakdown.

Statistical Control

Statistical control is not a natural state. All processes are under relentless attack from special causes.  Confusing common cause vs special cause variation results in one of two mistakes:

(1) The first mistake is to assume variation to be a special cause when it is in fact common cause. The mistake of over adjustment leads to more variation and time wasted looking for a reason for a defect when there is no single assignable cause.

(2) The second mistake is to assume variation to be common cause when it is in fact special cause. The mistake of under adjustment is a lost opportunity to find and eliminate a special cause. Special causes are not always present so it is best to start looking for them right away before the trail grows cold…

This distinction is not often obvious. A machine breakdown could be common cause. Walter Shewhart invented control charts to measure common cause variation and detect special cause events.