The exact u chart can be obtained using simple adjustments

Gemai Chen and Smiley W. Cheng

Statistics & Probability Letters, 1998, vol. 37, issue 4, 357-365

Abstract: The u chart for monitoring [lambda], the average number of defects per inspection unit, requires [lambda] to be large to have positive lower control limits, and the false alarm probabilities differ considerably from the nominal normal values. Based on nonlinear transformations, some alternative charts, such as the chart, can achieve large improvement over the u chart. We show in this paper that with very simple adjustments to the control limits of the u chart, one can achieve equal or even better improvement while still working on the original scale. In particular, using the very simple adjustments is equivalent to using the exact Poisson distribution to design a chart. Some guidelines for choosing an appropriate chart are given.

Keywords: Normal; approximation; Transformation; Cornish-Fisher; expansion (search for similar items in EconPapers)
Date: 1998
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