Common Causes vs Special Causes of Variance

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Designed to help those that are preparing to take the PMP or CAPM Certification Exam, each post within this series presents a comparison of common concepts that appear on the PMP and CAPM exams.

Common Causes vs 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.

Example

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.

Summary

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

It is important to understand:

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