We refer to these sources of variability that are not

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We refer to these sources of variability that are not part of the chance cause pattern as assignable causes of variation.
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A process that is operating in the presence of assignable causes is said to be an out-of-control process.
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Note: The terminology chance and assignable causes was developed by Shewhart. Today, some writers use the terminology common cause instead of chance cause and special cause instead of assignable cause.
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Processes will often operate in the in-control state for relatively long periods of time.
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However, no process is truly stable forever, and, eventually, assignable causes will occur, seemingly at random, resulting in a shift to an out-of-control state where a larger proportion of the process output does not conform to requirements.
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Statistical Basis of the Control Chart
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W here L is the ―distance‖ of the control limit from the center line
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This general theory of control charts was first proposed by Walter A. Shewhart, and control charts developed according to these principles are often called Shewhart control charts.
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Control Charts
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Control Chart is a device for describing in a precise manner exactly what is meant by statistical control; as such, it may be used in a variety of ways.
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Control Chart That is, sample data are collected and used to construct the control chart, and if the sample values of (say) fall within the control limits and do not exhibit any systematic pattern, we say the process is in control at the level indicated by the chart.
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Control Chart The most important use of a control chart is to improve the process .
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