Np-chart explained

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 = (\sum ^{m} _{i =1} \sum ^{n} _{j =1} \begin{cases)/(1) & if x _{ij} defective \ 0 & otherwise \end{cases}}{m}
Meanlimits:	n \bar p \pm 3 \sqrt{n \bar p( 1- \bar p)}
Meanstatistic:	n \bar p _{i} = \sum ^{n} _{j =1} \begin{cases} 1 & if x _{ij} defective \ 0 & 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.^[1]

The np-chart differs from the p-chart in only the three following aspects:

1. 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.

1. The number nonconforming (np), rather than the fraction nonconforming (p), is plotted against the control limits.
2. The sample size,

n , is constant.

See also

• p-chart

Notes and References

1. Book: Montgomery, Douglas . Introduction to Statistical Quality Control . John Wiley & Sons, Inc. . 2005 . . 279 . 978-0-471-65631-9 . 56729567 . dead . https://web.archive.org/web/20080620095346/http://www.eas.asu.edu/~masmlab/montgomery/ . 2008-06-20 .