# Calculating Cp and Cpk - Is my calculation correct?

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#### MarkHughes

I have attached a form for calculating Cp and Cpk, however it appears that my calculation for Cpk is actaully calculating Ppk. Can someone help my formula?

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#### Statistical Steven

##### Statistician
Staff member
Super Moderator
Re: HELP - Is my calculation correct?

If the values are consectutive, you can use the moving range to caluclate short term variability (s=movRbar/1.128). For your data the difference is very small.

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
Re: HELP - Is my calculation correct?

just to confirm Steven's response: we are assuming that your subgroup size was 1?

were these sequential pieces? or were they spread out in time?

and most importantly: are the values listed in descending order in the order in which they were produced? if not you can only calculate Ppk...

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#### MarkHughes

Re: HELP - Is my calculation correct?

Subgroup size is one. Yes they were sequential, numbered as they came out of the press.

What I'm really curious about is what is wrong with my calculation? The value I am getting in my sheet is what MiniTab gives me for Ppk. MiniTab is giving me a Cpk of 1.02

Is there something wrong with my calculation??

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#### Atul Khandekar

##### Quite Involved in Discussions
Re: HELP - Is my calculation correct?

Your sheet is calculating Ppk - uses Excel function STDEV.
Use the formula given above by Steven, and you will get the Cpk of 1.02.

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#### MarkHughes

Sorry, this may be a dumb question but, what does each part stand for in the equation? Or how do I calculate this in excel?

s=movRbar/1.128

s is..
mov is...
Rbar is ...

Thank you for everyone's help!!!

#### Statistical Steven

##### Statistician
Staff member
Super Moderator
Sorry, this may be a dumb question but, what does each part stand for in the equation? Or how do I calculate this in excel?

s=movRbar/1.128

s is..
mov is...
Rbar is ...

Thank you for everyone's help!!!
Here is a file with the calculations.

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• 53.5 KB Views: 172