# Questions re CpK and Ppk

Six Sigma – iSixSigma Forums General Forums New to Lean Six Sigma Questions re CpK and Ppk

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• #236103

Karim Mohsen
Participant

what is meant by Cpk is for short term, Ppk is for long term.? And
Is Cpk tells you what the process is capable of doing in future,while Ppk tells you how the process has performed in the past?

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#236114

Patrick Detroit
Participant

Hi Karim, I don’t consider myself an expert, but I’ll try to answer the best I can. How the denominators are calculated for Cp/Cpk and Pp/Ppk will answer your question. The denominator for Cp/Cpk uses a pooled Stdev or Stdev(within) that is calculated by obtaining the square root of the sum of subroup stdev’s. The denominator for Pp/Ppk is the Stdev(overall), which is the stdev of the entire study. Cp/Cpk is focused on how the process behaves based on your samples and Pp/Ppk focuses on the entire process. Hope this helps.

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#236115

Chris Seider
Participant
#236129

Patrick Detroit
Participant

Hi Chris,
Maybe you can answer a question I was recently asked. Why is Cp/Cpk a 2.0 ratio? I explained the math, that the USL-LSL is a measure of +-6 sigma (12 sigmas) over the denominator that is only 6 sigma. My follow on question… is there a reason we use a 2.0 scale? If we used a denominator of 12 sigma the scale would be just as understandable (0.17; 0.33; 0.5; 0.67; 0.83; 1.0). I tried looking online and have come up empty handed. Thanks

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#236130

Chris Seider
Participant

Cp is meant to tell you the ABILITY to produce good product so it’s just a ratio of 99.7% of the “normal” distribution relative to the specs.

Cpk is meant to give you an idea of how well your process is ACTUALLY doing relative to the closest spec to the mean–which is the highest portion of your defectives.

There isn’t a ratio of 2.0 between Cpk and Cp because they are comparing different things in the numerator. Good question. Last point…it’s always important to graphically look at how your process is doing relative to the specs.

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