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Np-chart

Field	Value
name	np-chart
proposer	Walter A. Shewhart
subgroupsize	n 1
measurementtype	Number nonconforming per unit
qualitycharacteristictype	Attributes data
distribution	Binomial distribution
sizeofshift	≥ 1.5σ
meanchart	Np control chart.svg
meancenter	n \bar p = \frac {\sum_{i=1}^m \sum_{j=1}^n \begin{cases} 1 & \mbox{if }x_{ij}\mbox{ defective} \\ 0 & \mbox{otherwise} \end{cases

In statistical quality control, the np-chart is a type of control chart 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{n\bar p(1-\bar p)}, 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.

References

Montgomery, Douglas. (2005). "Introduction to Statistical Quality Control". [[John Wiley & Sons]], Inc..

Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

quality-control-tools statistical-charts-and-diagrams

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