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On Properties of Poisson Q Charts for Attributes

Summary: The sensitivity of four tests on Shewhart Q charts and of specially designed EWMA and CUSUM Q charts to detect one-step permanent shifts of a Poisson parameter has been studied. The usual test that signals for one point beyond three standard deviations on a Shewhart chart is found to have poor sensitivity, generally. The test that signals when four out of five consecutive points are beyond one standard deviation in the same direction on a Shewhart Q chart is found to be a good omnibus test. The EWMA and CUSUM Q charts are most sensitive to detect this particular type of instability and are about comparable in overall performance.

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  • Topics: Quality Tools
  • Keywords: Attributes control charts,Cumulative sum control chart (CUSUM),Exponentially weighted moving average control charts (EWMA),Q chart (quality score),Shewhart control chart
  • Author: Quesenberry, Charles P.
  • Journal: Journal of Quality Technology