In developing process control charts, if special causes are present, then they are not representative of the true state of statistical control, and the calculations of the center line and control limits will be biased.

Question

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When special causes are present, they indicate that the process is not in a state of statistical control. In such cases, center line and control limits calculated using data from such periods may not be representative of the true state of the process. Therefore, it is important to identify and eliminate special causes before constructing control charts.

In Developing Process Control Charts, If Special Causes Are Present