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Discussion Thread Title: Cpk Formula seems off, need help!?
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Old 25th July 2017, 02:55 PM
howste's Avatar

Total Posts: 5,055
Re: CPK formula seems off, need help!?

If you look at the bell-shaped normal curve on Miner's post, you can see that part of the distribution is under the lower specification limit. The calculations you made are based on a sample from the total population of parts, which are estimates of the population. If you measured and plotted, for example, 1000 part readings from the same process without adjustment (assuming that it's normal) it should look like the normal curve and you would actually find that some parts are below the lower specification limit.

Statistically speaking, a Cpk of 1.0 would show up if your process center is 3 standard deviations from the closest of an upper or lower spec limit. A Cpk of 1.33 would be 4 standard deviations away, and a Cpk of 1.67 would be 5 standard deviations away. The farther you are from the spec limits, the higher the probability is that you will have no bad parts.
Thanks to howste for your informative Post and/or Attachment!

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