Supriya Ghosh (Editor)

Np chart

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Originally proposed by
  
Walter A. Shewhart

Quality characteristic type
  
Attributes data

Size of shift to detect
  
≥ 1.5σ

Rational subgroup size
  
n > 1

Underlying distribution
  
Binomial distribution

Np-chart

Measurement type
  
Number nonconforming per unit

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:

  1. The control limits are n p ¯ ± 3 n p ¯ ( 1 − p ¯ ) , where n is the sample size and p ¯ is the estimate of the long-term process mean established during control-chart setup.
  2. The number nonconforming (np), rather than the fraction nonconforming (p), is plotted against the control limits.
  3. The sample size, n , is constant.

References

Np-chart Wikipedia
(Text) CC BY-SA