Quality eLine Newsletter
January 2005
Vol. 7, No. 01

Quality Quiz

It’s a new year at Natural Butter Company, and that means yet another new quality manager. Marty Graw has come from a background in MIS and considers himself technically ahead of the game. In fact, he considers himself ahead of everything, undaunted by new responsibilities and confident that he will not only wow his boss and the quality technicians for whom he is responsible, but will also soon be the talk of the entire company, after he has quickly solved all of the packaging problems that have haunted the company. Marty Graw’s comprehension of statistical tools and concepts rests on his ability to put his hands on a copy of Practical Tools for Continuous Improvement (Graham and Cleary). He knows that if he is challenged by a statistical question, he can excuse himself, find an answer, and have a ready response.

He soon has a chance to demonstrate the efficacy of his just-in-time approach to understanding statistics, when his boss asks for capability analysis of the packaging process, focusing on the company’s Line 2. Marty Graw skims the chapter on capability, and offers his technicians, who have not done this analysis before, a description of capability, based on the seven-step approach used in the book.

These seven steps appear in sections entitled:

What capability is
When is it used?
What does it look like?
How is it made?
Getting the most from capability analysis

Stealing shamelessly from the book, Marty Graw presents its ideas to his technicians in a training session that he has launched. Sailing smoothly through the overview, he moves to an overhead representing the following figure from page 244 of the book:

Getting to the matter of “when is it used,” Marty Graw copies down the six questions related to conditions that must be true in order to do a capability study:

1. How does the system perform with respect to specification limits?
2. Does the specification consist of both upper and lower requirements?
3. Is the process in control?
4. Is the data in variable form?
5. Do the individual values form a normal distribution?
6. Has the data been collected over time?

Moving quickly through the conditions in order to avoid pauses where questions might be raised, he is ready to tackle “How is it made?” An example will help him present this concept:

At the suggestion of the Practical Tools chapter, Marty Graw decides to sketch the data’s distribution. He begins with a bell-shaped curve with the of 10.0 in the center.

He knows that he must put a scale on the axis, but has not yet read the next section, relating to estimated standard deviation. He decides instead to use the range , since he is more familiar with this statistic. Recalling from his skimming of the book something about three average ranges to the left and three to the right of the mean, he confidently shows the technicians how to move three units to the right of the mean, and three to the left. He adds this scaling to the curve:

With Marty Graw moving dauntlessly ahead, it is time to ask whether he is on the right track, particularly when it comes to scaling the curve. Select one of the following statements as accurate:

a) The graph is correct.

b) Marty Graw skipped a critical concept by not understanding estimated standard deviation.



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