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Utilizing Cpk in functional testing

 

 
 
 

Utilizing Cpk in functional testing

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When distribution and Cpk are analyzed based on results of five prototypes we get following.

Estimated distribution indicates that some test results would be outside acceptance limits. In this sample of five we did not have such units but what is the risk level? There is strict connection between Cpk and faulty units. With Cpk value 0.68 we get Yield estimate 97.9%. This means that in volume production about 2% of units will fail in this test. And the problem is that actually components are faultless and build is done just as designed. Quite a repair problem in production.

Of course sample of five units does not give very accurate Cpk. It can be used anyway to identify possible problem areas to be studied more. In this certain case next 72 samples gave more precise Cpk estimate and the value was even worse: 0.51.

In the chart in right we can see the Yield with various Cpk values. This figure is describing one-sided cases (mean not in the center point between limits).

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