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Estimating Common-Cause Sigma in the Presence of Special Causes

Summary: (This paper was presented at the Journal of Quality Technology Session at the 41st Annual Fall Technical Conference of the Chemical and Process Industries Division and Statistics Division of the American Society for Quality and the Section on Physical & Engineering Sciences of the American Statistical Association in Baltimore, Maryland, October 16-17, 1997.)In statistical process monitoring we test process data for departures from some model of common-cause variation, usually independent, identically distributed (iid) normal. One task in the set-up phase is to estimate the common-cause standard deviation. Most comparisons of estimators have assumed normal or contaminated normal distributions. The usual "trends and oscillations" argument for the standard moving-range estimator has not been adequately investigated, nor have other special-cause scenarios where the data cannot be reduced to an iid subset. I compare five old estimators and one new one in this context. A new proposal for retrospective analysis is based on the results of these comparisons.

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  • Topics:
  • Keywords: Moving range control charts,Outliers,Maximum likelihood estimate (MLE),Median,Trends
  • Author: Boyles, Russell A.
  • Journal: Journal of Quality Technology