Why Special Cause and Common Cause Matter

There are many approaches to problem solving – 8D, Kepner-Tregoe, Six Sigma, Seven Step and Hypothesis Testing to just name a few.  A common goal of these methods is to get to the “root cause.”  Unless you fix the root cause, you are just treating a symptom of the problem and the cause remains to cause more problems.

But what is often over-looked is the nature of the root cause.  Is it a “smoking gun” where there is an unusual condition that is a direct link to the problem condition?  We often refer to this as a special cause.  Or is it a systemic problem that is caused by a several factors coming into an alignment that created the problem?  This condition is referred to as a common cause.  That nature of a successful solution is fundamentally different depending upon the answer to those questions. 

So before jumping to a solution to your problem, make sure you understand if it is a special cause or common cause.  The solution approach changes depending upon which type caused the problem.

Special Cause

Special causes are inherently unpredictable.  There are only two effective approaches to eliminate special causes.  The first is to avoid the conditions that can enable them.  For instance, if you find that a special cause problem occurred in a business process when you changed suppliers, don’t change suppliers.  If you can’t avoid the condition, then you need to put an “early warning” signal into your process so that you are aware when a special cause condition has occurred and you can quickly stop and contain the problem.  Let’s say you have traced a problem within a business process to a particular piece of equipment that failed to operate correctly and created several large batches of poor quality output.  You can put a monitor on that piece of equipment so that if it fails again, you are immediately notified and can stop the process and fix it.   

Common Cause

While special cause problems are unpredictable, common cause problems are totally predictable.  They occur when a system or process is pushed to provide an output that is better than what it is capable of doing.  The system or process may get lucky and occasionally deliver an acceptable result, but it is unable to reliably deliver that result.  When this is the nature of the root cause, the only options are to improve the system or go to a totally different system.  Both of these are often expensive and time consuming.  This condition often occurs when a system that was design for one purpose is used for a different purpose.  It may have performed satisfactorily in the initial pilot run when everyone was watching everything very closely, but it is not able to sustain good performance in normal operating conditions.

So why do we care whether it is special cause or common cause?  Well, doing a common cause system change to a special cause problem doesn’t prevent the special cause condition from happening again.  And doing a special cause change to a common cause problem will often make things worse since tampering with a system often reduces its reliability.   Special cause solutions focus on isolating and controlling the “smoking gun.”  They can be quickly implemented.   While common cause solutions require a system upgrade or total change in the business approach.