Vol. 7, No. 03
Marty Graw had a small, but significant breakthrough last month in his efforts to vindicate the capability lion that he has faced at Natural Butter Company. As we saw last month, he has learned how to calculate the estimated percent of out of spec weights in the packaging problem he is grappling with.
Marty is now ready to leap to the final step in calculating the capability index. Referring again to Practical Tools for Continuous Improvement (Graham & Cleary), a book that he props on his desk like a cookbook, he flips to pp. 252-256 to begin his first calculations. He has determined that these will be the Cp. Cp for this situation would be expressed in the following formula:
"Hmmm," he reads: "'Cp is an index used to assess the width of the process spread in comparison to the width of the specifications.' Exactly what I need!" He takes the memorized definition, plus figures he has found on page 253, to his boss to demonstrate his calculations:
Marty Graw is on a roll, elaborating and expatiating in his new confidence. Pointing to case A, he emphasizes the importance of reaching a point at which every weight of every butter package would be within specification. “Ideally, of course,” he says, pointing to case C, “upper and lower spec limits would lie far beyond the package weights. That,” he adds with a flourish, “would be nirvana.”
After his dramatic and confident remarks about capability, Mr. Smarty Pants now insists that the Cp of 1.17 means that everyone should be pleased with the process. He is challenged when his boss demands a reason that 2.28 percent of the packages on the line are of the wrong weight. “How can this be, if the Cp is greater than 1?” he asks.
What has happened here? Are Marty Graw’s observations about the relationship between package weights and spec limits correct? Does he know the meaning of the word ‘nirvana’? Find the answers to these pressing questions...
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