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No. of Defects on Six Sigma Scale

In addition to protecting against joint events, the increase capability afforded by the six-sigma goal also protect against problem that has the technical name of shifting of the mean.In fact, protecting against this problem is so important what Motorola built their six-sigma model on mean-shifted performance. An example will serve to show the impact of the shifting of the mean. Take a mean-centered process that is performing with a capability index of 1.0. The upper and lower specification limits will each be 3σ from the mean. The likelihood that random variation will produce a result more than 3σ from the mean of a stable process is merely 27 changes out of 10,000 opportunities, or 0.27 percent. But what happens if the process changes?

The process is upset and being yielding products with their mean shirted by one standerd deviation.The shifted process now produces 10 times the number of defects, 272 percent. Alternatively, had the process being performing at the six-sigma goal, the same shift of one standard deviation would have merely increased defects from the immeasurably 3.4 ppm to a nearly immeasurably 233 ppm.

Predicting performance for various example processes
Individual performance each step or component
SigmaCpk%
Accept
Defects (ppm)
40.8599.386,200
51.1799.99823
61.5099.99973.4
Resulting performance ppm defects as a function of number of steps or components
481362
24,60048,50095,000180,000
92184368736
142754109

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