Alternative Process Capability Indexes

Alternative Process Capability Indexes - fashion based on...

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Alternative Process Capability Indexes Researchers are continually studying process capability and evaluating new ways of measuring it. One alternative is to adjust C p by a factor (1 - k ) as follows: where k = 2|mean - target|/tolerance. When the sample mean is equal to the target, k = 0 and C pk = C p . As the sample mean deviates from the target, the absolute difference between them increases and k increases. Specification limits are used only to determine the tolerance; thus the focus of this measure is on the target value rather than on acceptable specification limits. Another index that has been proposed is This measure also accounts for deviations from the target value in a quadratic loss
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Unformatted text preview: fashion based on the Taguchi loss function. To illustrate the differences in these indexes, suppose that UTL = 8, LTL = -8, target = 0, = 3, and = 1 (see Figure) 1 . Then, Now consider another case. Suppose that UTL = 2.7, LTL = -2.7, target = = 0, and = 1 (see Figure 2). Then Figure 1 Off-Center Process; C pk = 1.66, C pm = 0.84 (5E Fig 9.21) Figure 2 Centered Process; C pk = 0.9, C pm = 0.9 (5E Fig 9.22) 1 Kalyan Kumar Chowdhury, E. V. Gigo, and R. Raghavan, Quality Improvement Through Design of Experiments: A Case Study, Quality Engineering 12, no. 3 (2000), 407416. Reprinted from Quality Engineering , courtesy of Marcel Dekker, Inc....
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