April  2003

Vol. 5, No. 4


Quality Quiz
Another quiz from Professor Cleary

Dinah Thyrst, quality manager for Brush Broom, longs for a black belt. Not only might this status assure her promotion to production manager, but she also envisions herself in the garb of a mighty warrior, the black belt tied loosely but gracefully around her waist. “I’d wear my black belt all the time,” she thinks. “After all, black is a neutral color.” Dinah realizes that the key to her promotion lies in securing a million-dollar savings on something, and the key to doing that lies in being named a black belt.

One area of the company has been experiencing dramatic losses. Part #5290F, a metal fastener for large push brooms, has a high profit margin, but the cpk on a critical dimension is .5, and the scrap rate is high. It occurs to Dinah Thyrst that if she can get the critical dimension of this part under control, this alone might generate significant savings for the plant. Since she has recently attended a seminar on SPC and has read statistical textbooks relating to the use of control charts, she decides to put into practice some of the theory that she has garnered from these sources.

One of the books that she has consulted recommends X-bar and S charts as more useful than X-bar and R charts in some situations, so Dinah decides to impress her plant manager with this approach. To learn what X-bar and S charts are, go to http://www.qualityadvisor.com/sqc/x-barsigma.htm

Dinah creates an X-bar and S chart on data collected for Part 5290F, and notices that the S section of the chart has control limits that vary greatly.

She has no explanation for this variation (hint: notice changing sample sizes), and while she is examining the chart for clues, the plant manager comes into her office. Looking over her shoulder at the chart, he wonders aloud about the variation in the control limits. Dinah Thyrst’s heart sinks, for she has no clue why this is the case. Seeing her elusive black belt possibilities floating out the window, she asserts quickly that the “S” in X-bar and S charts means “sliding.” “This kind of chart allows control limits to slide up and down,” she adds.

Any chance that this is indeed the case?

Yes. Dinah Thyrst has slid into home base. S charts do indeed reflect sliding control limits.

No. The black belt in her future has slipped to her knees. She must have made an error with her data.

 


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