Variation

One of the most important concepts in quality improvement is variation.

"If I had to reduce my message to management to just a few words, I'd say it all has to do with reducing variation." - W. Edwards Deming

Knowing how to interpret variation means knowing the difference between common and special cause variation.

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Exercise:

Write the letter 'a' eight times using your dominant hand. What do you see? Are they all identical? Why not?

Possible causes:

 pen used
 fatigue of the writer
 paper
 caffeine level of writer
 smoothness of surface
 angle of writer's hand
 etc.

Would it make sense to circle one of the a 's and ask , "Why it is different?"   The answer is "No!"   That particular 'a' is part of a system that includes all the reasons listed above (and many others).   This is called common cause variation .  It is variation that is part of the system.  Variation is inherent in all processes!

Now write 3 a's with your dominant hand, 3 with your non-dominant hand, and 2 more with your dominant hand.

Which ones are obviously different?  You can circle the radically different a 's and say, "Something out of the ordinary occurred here!"   (The answer is that you switched your hand.)  This variation is called special cause variation.

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Types of Variation

Common Cause Variation

 Variation that is inherent in the process.
 It is produced by the interactions  among the variables of the process.
 This collection of variables and their interaction is called the system of common causes.
 Management may assign teams consisting of internal suppliers, the owner of the process, customers, and others who can contribute knowledge about the common cause system to improve the process.

Special Cause Variation

 Variation in the process that is assignable to a specific cause or causes.
 This variation arises because of special circumstances.
 An individual worker may work on this source of variation.
 Management may assign a team to work on removing special causes, but team composition will likely be different from a team working on reducing common cause variation.

Two Systems of Variation

Special and Common Cause System

Actions to take:

 Immediately try to understand when a special cause occurred.
 Determine what was different when the special cause occurred.
 Identify ways to prevent the special cause from recurring, if understood.

Actions to avoid:

 Tampering or responding to change in a process and taking action without understanding the nature of the variation in the process.
 Doing nothing at all.
 Making fundamental changes in the process.

Common Cause System

Actions to take:

 Understand that the process has an inherent capability which will not change unless the process is changed.
 Identify aspects of the process that contribute to the common cause system.
 Determine which aspect of the process to change in order to reduce variation or adjust the middle value of the process.
 Plan the process change.

Actions to avoid:

 Doing nothing at all.
 Trying to interpret individual occurrences of the process or to explain the difference between those that are high and low.

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