Common Cause and Special Cause Variation
Though sometimes we may refuse to recognize it, the world is a full of variation, even in the things we think or believe are constant. For example, my wife has been known to say, “you always do…”
or “you never do…”, to which I retort, I am human and I am not that repeatable. I say that, but it is not just humans that are not so repeatable. Everything has variation, and understanding this variation, is important for product development, project management, manufacturing, product testing and so much more. To master this work, we need to understand this variation as completely as possible.
Common and Special Cause Variation
Shewart is credited for developing the concepts of special and common cause . Special cause variation, are variations that are outside of the expected (intermittent) range of possibilities. Common cause variation, are the variation expected, we know about these, these are predictable, provided we have put some effort into learning about this variation. For example, the thickness of a piece of rolled out steel plate will have variation that is due to the material and process that delivers. These are common cause, these variations are there by dint of the work, material selected and the processes that deliver the steel plate. These variations will exist in the product and processes we use to develop and manufacture the product, and it is the in part the management’s job to help understand these variations and the impact and implications on the work. It should be apparent, that not all variations are created equal, meaning, we do not need to respond equally to all variation, some are more painful that others. When we design the product and the processes, we have determined the rages of variations that we are willing to accept, that is, variation within this defined range still means success. The product functions with that variation, and there are no perceived quality issues from the customer.
We get variation in the product due to tolerances of the constituent parts, both mechanical and electrical. Our design has to account for the individual variations and the resulting interactions, and is the reason why we may choose 1% resistors for some parts and 10% for others. For mechanical parts, we define the acceptable variation via drawing tolerances in plus/minus form. How we handle this variation to some degree will influence the cost of the product.
The processes that create the production level product also have variation (also in the prototype phase but not important for this post). Generally speaking, the tighter we control our production process variation, the more costly the manufacturing. Not understanding what we truly need to produce the product meeting the quality expectations, means either too costly (in time and money) a process to deliver, or a process that is incapable, that is producing product that will not stand up to the customer’s expectation and cost in poor quality and warranty dollars.
Variation is here, it is not going anywhere. We should understand the product use, environment and how we manufacture (process) it understanding the variation and the impact on the product quality. Knowing this baseline and being able to differentiate from the variation that we expect and that which is the unpredictable shot from nowhere, allows us to quickly respond to these situations without becoming desensitized by responding to the common cause variation believing this variation may lead to bad outcomes.
 Taguchi, G., Chowdhury, S., Wu, Y., Taguchi, S., & Yano, H. (2005). Taguchis quality engineering handbook. Hoboken, NJ: Wiley. page 1446