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Departments: SPC Guide

Photo: Michael J. Cleary, Ph.D.


Capability Competence
Simsack strikes out.

Michael J. Cleary, Ph.D.


ock DeBote, CEO of Greer Grate & Gate, has hit bottom. Under pressure to produce profits and increase earnings, he's taken a few small shortcuts--such as reporting several million dollars as capital expenses--by mistake, of course, as he's always considered himself an honest man. He'll admit, however, that he's enjoyed the accolades that have come his way when quarterly reports showed surprising earnings, not to mention the substantial bonuses he's earned for keeping the company profitable. At home, DeBote's also enjoyed the time spent with his children on his new yacht and the vacations in Aspen where they all learned to ski while staying in his vacation house there. "I'm a family man," he insists when he's accused of wrongdoing, and he intends to use this as his main line of defense at his upcoming trial.

Quality Manager Hartford Simsack, having spent time in prison prior to being hired by DeBote, has empathy for his boss's situation--especially because DeBote is also his brother-in-law. But he worries most about his own vulnerability with respect to his limited understanding of statistics and his propensity to fake his knowledge about a variety of concepts related to statistical process control. DeBote's replacement, Newt S. Tament, whose integrity is beyond reproach and whose understanding of SPC has produced a record of quality successes in several companies, worries him. "I'm in trouble," Simsack says to himself.

Determined to impress his new boss with his statistical prowess, Simsack organizes a training session for the entire quality department. He plans to dazzle the participants with his terminology, charts and silver-tongued explanations of complex statistical processes. And, as a courtesy, he'll invite Tament to the session, but he'll schedule the training for a time when he knows his boss will be unable to attend, at the same time there's a planning session at the company's world headquarters.

Beginning what he hopes will be a scintillating presentation about capability analysis, Simsack glances around the room and realizes his boss has somehow made arrangements to be at the session and is sitting in the back of the class. "The normal distribution should be--well, normal," Simsack begins, hoping that by stating the obvious, he'll warm up his group a little. He's prepared a canned lecture that he copied nearly verbatim from a statistics textbook.

After he correctly points out that a process must be in control before beginning capability analysis, he makes his observation about normal distributions and proceeds immediately to chi-squared analysis, hoping that no one will ask questions; he knows only what he's read and memorized from his book. He astounds himself with what he can actually remember, and his confidence soars when no one, including the CEO, asks questions. So he proceeds to offer an exercise for the students. It's taken directly from his statistics book and examines the following data:

1. Cp = 1

2. Cpk = -1

3. = positive number

4. = negative number

Endeavoring to demonstrate his interest, one of the participants asks whether the process is capable. Simsack says, "Of course it is. If the Cp equals 1, the process is producing 99.73 percent good--that is, within spec limits--parts."

Is Simsack's response true or false?



Simsack's response is false.

If the conditions listed are in place, the distribution of Xis would be below the lower spec limit. A visual demonstration of this may be helpful.

A process that is in control, with Xis following a normal distribution, will have these parameters:

1. LSL = 6

2. USL = 12





This can be drawn as follows:


Cp = 1

Cpk = -1

Although Simsack is wrong in this case, he's learned a great deal about finding someone to blame for his mistakes. His strategy, therefore, is to insist that he knew the right answer; the unwitting seminar participant, however, had asked the wrong question.

About the author

Michael J. Cleary, Ph.D., is a professor emeritus at Wright State University and founder of PQ Systems Inc. He has published articles on quality management and statistical process control in a variety of academic and professional journals. His Web site is www.pqsystems.com. Letters to the editor regarding this column can be e-mailed to letters@qualitydigest.com.