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Discussion on Mean Value Control Boundary of Shewhart Control Charts |
FAN Qiao-cheng |
Shandong Electric Power Research Institute, Jinan, Shandong 250002, China |
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Abstract The Shewhart control chart can be used to control the measuring process whether it is under the state of statistical control. But it is considered that the confirmation for the control upper and lower limits of the mean value is improper. For the reason that the upper and lower control limits is established from standard deviation of measuring data under repeated conditions. And then, using it to control the mean value's variations character of sub-groups, in fact, the former cannot control subgroup variation due to the condition is different. Other way is described to confirm the upper and lower control limits which is based on measurement assurance programs(MAP). This method is to use sub-groups' standard deviation obtained from repeated conditions to setting the control limits, i.e. the dispersion of mean value within the subgroup to control the mean value. It has proved that the determination of the mean value controlling boundary via Shewhart control chart is improper by means of comparing the Shewhart with MAP method during analyzing the same experimental data of the DC resistance, DC voltage and electricity meter test equipments.
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