# Calculating the Overall Cpk

Six Sigma – iSixSigma Forums Old Forums General Calculating the Overall Cpk

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• #36059

Jude
Participant

If a certain process has several parameters (with different specifications) being measured by Cpk, how do we calculate the overall Cpk? Is getting the weighted average an acceptable approach?

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#102990

Bob J
Participant

Jude,
No, a weighted average would not be mathematically valid….
Right or wrong we consider the overall Cpk of a process to be the lowest Cpk value of a critical parameter…  The logic we use is that the process is only as good as the weakest critical parameter.
Hope this helps…
Best Regards,
Bob J

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#102993

Reigle Stewart
Participant

Jude: Convert each Cpk to a standard normal deviate;
i.e., Z=Cpk*3. Next, you must convert Z to equivalent
yield (in Excel use normsdist(Z)). After this, you need to
multiply all of the yield values to get the rolled throughput
yield … Y.rt = Y1 * Y2 * … * Yn, where n is the number of
yield values (i.e., the number of Cpks you converted).
Then normalize the rolled yield by computting Y.norm=
Y.rt^(1/n). Finally, convert Y.norm to a Z value (using
Excel) and then divide by 3 to get the final answer …
Cpk.norm = Z.norm / 3. The resulting normalized Cpk is
“kind of like an average” but is statistically valid. Kind
Regards, Reigle.

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