# p-chart Cpk

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• #29851

Suwandi
Member

We can calculate the cpk and cp for x-bar control chart.. can we do the same thing for p-bar? what is the formula? is cp=1-pbar can represent the calculation?
Grazie

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#77170

Gabriel
Participant

Yes. Ussually p bar (the proportion of non conforming parts in the process) or 1 – p bar (proportion of conforming) is taken as the capability index. Only that it is not called Cp nor Cpk. Just p bar or 1 – p bar. (Ref: AIAG SPC page 107)

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#77181

Sridhar
Member

Hi,
If you more than 30 data points ,you can calculate the standard deviation as sqrt(p*(1-p)) ( based on central limiit therom defeinition). Rest of the calculations will remain same, i am not sure whether this method of calcuating Cp is correct or not.
thanks

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#77193

Gabriel
Participant

The standard deviation calculated as you said will be the standard deviation of the proportion of bad parts. For example, if you are chaecking a diameter but only record good/bad, then it will be the standard deviation of the proportion of parts with “bad” diameter. But to use the Cp/Cpk formula, wht you need is the standard deviation of the diameter itself.
What you can do, at best, is (in the previous exampe) to distinguish the bad parts in “too big” and “too small”, then you have a % big and a % small that you can compare with the Z(upper) and Z(lower) for a normal distribution, and with those Z calculate Cp and Cpk (if you have a one sided tolerance you will have only one Z and only Cpk).

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