np-chart

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np-chart
Originally proposed byWalter A. Shewhart
Process observations
Rational subgroup sizen > 1
Measurement typeNumber nonconforming per unit
Quality characteristic typeAttributes data
Underlying distributionBinomial distribution
Performance
Size of shift to detect≥ 1.5σ
Process variation chart
Not applicable
Process mean chart
Np control chart.svg
Center line
Control limits
Plotted statistic

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 , where n is the sample size and 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, , is constant.

See also[edit]

References[edit]

  1. ^ Montgomery, Douglas (2005). Introduction to Statistical Quality Control. Hoboken, New Jersey: John Wiley & Sons, Inc. p. 279. ISBN 978-0-471-65631-9. OCLC 56729567. Archived from the original on 2008-06-20.