Vol. 7, No. 06
Marsha Mallow and Maura Thesame are like premier chefs when it comes to quality. They consult the cookbook—in this case, Practical Tools for Continuous Improvement (Graham & Cleary)—when they think they need to, but they add their own pinches of salt when they feel like it.
The cookbook has made them aware of the need for a process to be in control before an accurate capability analysis can be undertaken. “We can do that,” Mallow says; Thesame adds, “We can make a control chart to check whether the butter packaging process is in control.” Glancing at the example in their book, they see that it involves 25 samples of 5 each, so that’s the recipe they decide to follow.
Checking five packages of butter every hour, they collect a substantial amount of data and chart it, using CHARTrunner to create the following control chart:
“Great chart,” says Mallow. “It’s in control,” Thesame adds. “Let’s do a presentation to managers,” says Mallow. “We can import the chart to PowerPoint,” says Thesame.
Process managers who see their presentation are duly impressed with the stability of the system they are describing, but one of them wonders about the sampling process. “Why did you pick 5 for your sample size, rather than 4, or 6, or some other number?” Mel O. Dius, assistant plant manager, asks.
Unfortunately, the dynamic duo has left the cookbook back in the kitchen, so the answer to this question momentarily escapes them. Undaunted, however, Marsha Mallow remembers an old adage, “When in doubt, say something anyway,” and is emboldened enough to explain that 5 is selected because it is the square root of 25, the number of samples in their control chart.
Is Marsha’s answer “out of the book,” or has she added too much salt to the stew?
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