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statistical quality control Statistical process control (SPC) or statistical quality control (SQC) is the application of statistical methods to monitor and control the quality of a production process. This helps to ensure that the process operates efficiently, producing ...
, the np-chart is a type of
control chart Control charts is a graph used in production control to determine whether quality and manufacturing processes are being controlled under stable conditions. (ISO 7870-1) The hourly status is arranged on the graph, and the occurrence of abnormalit ...
used to monitor the number of nonconforming units in a sample. It is an adaptation of the p-chart and used in situations where personnel find it easier to interpret process performance in terms of concrete numbers of units rather than the somewhat more abstract proportion. The np-chart differs from the p-chart in only the three following aspects: #The control limits are n\bar p \pm 3\sqrt, where n is the sample size and \bar p is the estimate of the long-term process mean established during control-chart setup. #The number nonconforming (np), rather than the fraction nonconforming (p), is plotted against the control limits. #The sample size, n, is constant.


See also

* p-chart


References

{{DEFAULTSORT:Np-Chart Quality control tools Statistical charts and diagrams