Vol. 7, No. 07
Marsha Mallow and Maura Thesame continue to learn about statistics, as they wend their way through Natural Butter’s slippery slope of quality improvement.
Referring to Practical Tools for Continuous Improvement (Graham, Cleary), they prepare yet another in an endless stream of PowerPoint presentations designed to enhance their position as quality experts by offering an instructional session on X-bar and R charts.
Using the following chart, Mallow and Thesame use their laser pointer to indicate relevant points about the mean and the range, sweeping across upper and lower control limits as part of the landscape.
“How does the software determine control limits?” someone in the darkened room asks.
“Good question,” says Marsha Mallow (always a good avoidance response, giving time to think about the good question).
“It’s a complex process,” mumbles Maura Thesame, adding that it seemed to be time to take a short break before resuming instruction.
If the two instructors race to their copies of Practical Tools, they will find the formulas by which control limits are calculated—provided that they know what they’re looking for—in time to respond to this good, complex question. Here’s what they will find on pp 80-81:
They put the formula on the whiteboard, and resume the session.
"Of course," begins Mallow, " represents the process average," reading from the book." Or...
She copies the next formula onto the board:
("K is the number of samples," Maura Thesame pipes up, "and is the average range."
"But what is ?" Someone in the audience ventured.
Knowing that they could not take another break so quickly, Maura Thesame tried to explain the relationship of to SPC as an analogy. "It's sort of like steak sauce is to a steak," she manages.
Of course, all analogies ultimately break down. What about this figurative example?
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