If something is constant, it is most definitely dead. Variation is a natural phenomenon and it is observed as a fluctuation in the output of the machine / system / process. It is most commonly denoted by σ (sigma). Following are the two types of process variations.
Common Cause Variation - This is the random variation and is natural for the process. As name indicates it is “common” to the process. It is the desirable variation in the process. It is also called as within-group variation or inherent variation. Since it is inherent, there is no assignable cause for this variation.
Special Cause Variation / Black Noise - This is the non-random variation and is not part of the process. As name indicates it is “special” to the process. It is also called as Black Noise. Since it is special, there is always an assignable cause for this variation.
An application oriented question on the topic along with responses can be seen below. The best answer was provided by Arunesh Ramalingam on 28th September 2017.