It’s safe to say that all quality practitioners are familiar with the control charting terms “common cause” and “special cause” variation. However, how many have really thought about their interpretations and associated action plans which could also add to variation? To examine this line of thought, we need to review what I’ll call the Shewhart approach and then the Deming approach.
In the early 1920s, Dr. Walter A. Shewhart of Western Electric Company developed a theory that there are two components to variation: an inherent component from random variation he called chance-cause variation and an intermittent variation due to special cause which he referred to as assignable cause variation.