Assignable Cause Variation

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Start by identifying everything that can affect your product. It is reflecting about everything that may affect product quality, which is important.

All processes are subject to variation, which may be classified as random or chance variation (common cause) or assignable cause variation (special cause).

Many believe that an out of control process produces defective parts. The control chart in the figure below illustrates points outside of both the lower control limit (LCL) and the upper control limit (UCL), indicating that the process is out of control.

However, the points remain within the specification limits.

Special cause variation, on the other hand, is unpredictable, sporadic, or unstable variation.

It is the result of a specific assignable cause, for example Machine (Equipment), Man (Operator), Material, Method, Milieu (environment).

Deming’s second principle: “It is management’s responsibility to know whether the problems are in the system or in the behavior of the people.” There are two types of process variation: The goal of statistical process control is to understand the difference between these two types of variation—and to react only to assignable cause variation.

Processes that show primarily common cause variation are, by definition, and running as well as possible.


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