Common causes and special causes

Mayo Clinic video for common and special cause

What type of variation should we left to chance? Presentation with charts  LINK

From another aspect LINK

The basic rule of statistical process control is:

Variation from common-cause systems should be left to chance, but special causes of variation should be identified and eliminated.

Common Causes of Variance versus Special Causes of Variance

These quality definitions refer to variances or variations in systems, processing, or tested results.

Common Causes of Variance

Common causes are the normal, expected variances that occur. Common causes are predictable and are not considered unusual.

On a control chart, common causes of variance would be indicated by the random points within the control limits.

Common causes of variance are also known as “random” causes.

Special Causes of Variance

However, special causes of variance are those causes that are not predictable or inherent in a system. Special causes are usually related to some type of defect.

On a control chart, special causes are represented by points beyond the control limits or as non-random points within the control limits.

Special causes of variance are also known as “assignable” causes.


We recently had the pleasure of doing a home exchange with a couple in Paris, France. Unlike Phoenix, Paris has a phenomenal public transportation system. By using the Metro, we were able to travel all over the city and visit a lot of great locations.

At the Metro stations, there is a reader board next to each track telling when the next train will arrive.

The trains arrive anywhere from four to five minutes apart. These variances would be considered from common, or random, causes.

However, on my birthday (a particularly cold night), we go down to the Metro station to get our train back to our arrondissement, check the reader board, and see that the train is delayed by 25 minutes due to a mechanical failure.

This variance would be considered from a special, or assignable, cause.


The causes of variance are commonly tested topics on both the PMP and the CAPM exams.

It is important to understand:

• How they differ
• What are their alternate names
• How they may be represented on a control chart


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: