Special Causes of Variation is when something happens to cause a variation in the output (Y) that is unusual and is not consistent or constant. Special causes of variation are often referred to as assignable causes because they can be eliminated with a response or a solution to individual variations. Control Charts can be used to differentiate between common causes and special causes of variation.
Example: In the photo above three arrows hit the target dead center. One arrow hit the target high and to the left. The arrow that landed high and to the left is an example of a special cause of variation.