April 2001

Vol. 3, No. 4


Quality Quiz from Dr. Cleary

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Here we have a case of unfortunate terminology. "Out-of-control" is a pejorative term to many, implying that something is wrong with the process. But an individual moving range chart can measure processes in which one hopes that the data will have an upward trend (e.g., value of your 401K account). In this case, a point outside the upper control limit reflects a positive development, even though it is considered out of control.

A run above the mean, or any upward run, would likewise be desirable. But again, it would be labeled "out of control." In the lexicon of statistical process control, expressions such as "stable" or "unstable;" "predictable" or "unpredictable;" "consistent" or "inconsistent;" might be considered more accurate in describing a process. But the key lies in not only being able to recognize an out-of-control situation, but even more important, to understand the process well enough to know whether the situation is a positive or negative development.

Copyright 2001 PQ Systems.

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