AL Being out of control and
being out of specification are two entirely different concepts.
That is, on the average, about every fourth point you plot on each control chart will trigger one of these 100 rules and you’ll think there’s an out- of-control situation when there really isn’t. You’ll waste too much time looking for assignable causes that aren’t there. My advice is to forget all these specialized rules and stick with the three-sigma rule and run-of- whatever rule only.
“So,” you say, “if my process is out of control, that’s the same as being out of specification. I’d better shut it down.” Not necessarily. Being out of control and being out of specification are two entirely different concepts (hint: look at Figure 1). We’ll talk about this more in the next column.
References 1. Bidlingmeyer, B.A.; Deming, S.N. et al. Retention mechanism for
reversed-phase ion-pair liquid chromatography. J. Chromatogr. 1979, 186, 419–34.
one of these decisions will give a wrong answer is much bigger than you might think. The relevant equation is:
(1.1)
If, for example, d = 100 rules are used, each with a risk α = 0.003 of being wrong, the overall “experiment-wise” risk (αEW
) is equal to: (1.2)
2. Wheeler, D.J. Advanced Topics in Statistical Process Control: The Power of Shewhart’s Charts; SPC Press: Knoxville, Tenn., 1995.
3. Shewhart, W.A. Statistical Method from the Viewpoint of Quality Control; Deming, W. Edwards, Ed.; The Graduate School, The Department of Agriculture, Washington, DC, 1939.
4. Small, B. Ed. Western Electric Statistical Quality Control Handbook; Mack Printing Company: Easton, Penn., 1956.
Dr. Stanley N. Deming is an analytical chemist who can be found, as he says, masquerading as a statistician, at Statistical Designs, 8423 Garden Parks Dr., Houston, Texas 77075-4731, U.S.A.; e-mail:
statisticaldesigns@gmail.com;
www.statisticaldesigns.com
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APRIL 2016
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