On Properties of Poisson Q Charts for Attributes

Abstract

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.

• Attributes Control Charts
• Cumulative Sum Control Charts
• Exponentially Weighted Moving Average Control Charts
• Q Charts
• Shewhart Control Charts

Additional information

Notes on contributors

Charles P. Quesenberry

Dr. Quesenberry is a Professor of Statistics. He is a Fellow of ASQC.