What distinguishes “assignable causes” from “common causes” of variation?

The distinction between assignable causes and common causes of variation is fundamental in quality management and Six Sigma methodologies. Assignable causes, also known as special causes, are identifiable factors that lead to variation in a process. These causes can often be traced back to specific events or actions that can be fixed or eliminated to reduce variability. For example, a malfunctioning machine that causes defects in a manufacturing process is an assignable cause that can be addressed directly.

In contrast, common causes, or inherent causes, are the natural, background variations that are present in a stable process. They are part of the system and are not due to any identifiable event or specific factor. Common causes are essentially the result of the way the process operates and will always exist unless the process itself is fundamentally redesigned or improved.

Understanding this distinction is critical for quality improvement because it helps practitioners determine the appropriate approach for addressing variation in a process. If the variation is due to assignable causes, targeted interventions can be applied. However, if it’s due to common causes, broader systemic changes may be necessary.

The option highlighting this distinction correctly identifies that assignable causes can be traced to specific factors, while common causes are inherent to the process. This understanding is essential for effectively utilizing Six