The control limits in the x-bar chart are set so that 95 percent of the values will fall inside the control limits when there is only common cause variation.

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The control limits in the x-bar chart are typically set at ±3 standard deviations from the process mean, which corresponds to approximately 99.73 percent of the values falling inside the control limits under the assumption of normal distribution and only common cause variation present.

The Control Limits in the X-Bar Chart Are Set So