# Need help calculating actual Ppk - Result of % GR&R=11.8% when conducting MSA

J

#### johnny xu

Suppose we get the result of % GR&R=11.8% when conducting MSA, and then calculate PPK=1.67.

Who tell me actual PPK value is above or below 1.67 ? and why?

THX

#### Miner

##### Forum Moderator
Staff member
The relationship is between the variances (s^2) and is shown by this equation:

S^2 (observed) = S^2 (actual) + S^2 (measurement)

This may be restated as:

S^2 (actual) = S^2 (observed) - S^2 (measurement)

Take S^2 (observed) from the capability study and S^2 (measurement) from the MSA to calculate S^2 (actual). Use this to calculate the actual Cpk/Ppk.

Note: if reporting this to a customer, be truthful about what you did.

J

#### johnny xu

Firstly I’d thanks for your help.

But I have two part question.

a. Regarding formulate you provided , which book illustrate it? AIAG MSA? Or others?
b. Take my question for example , as you mean, PPK calculated value 1.67( observed value) ----is right? If wrong, can you give me a detailed example or figure to explain this formulate---which is value observed ? value measured?

Thanks a million

#### Miner

##### Forum Moderator
Staff member
I do not have my copy of the MSA manual at hand, but I believe that the formula may be found in it. I know it may be found on page 5 of Dr. Wheeler's An Honest Gauge R&R Study.

The true Ppk will be greater than 1.67. The MSA manual has a chart that shows the effect of measurement variation on capability. Or you can use the equation that I provided and calculate it directly.

#### Rameshwar25

##### Quite Involved in Discussions
The explanation is given in MSA IVth Ed (Pg 199-202).

regards
rameshwar

J

#### johnny xu

Hi miner,

Thanks for attachment you provided , I read it carefully on page of 5 , but didn’t find it.

J

#### johnny xu

The explanation is given in MSA IVth Ed (Pg 199-202).

regards
rameshwar

Thx , can you share MSA (4th edition)

Last edited by a moderator:

it's here.

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#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
One thing to remember is that the formula is a mathematical truth and doesn't require the 'validation' that a 'standard' may confer. It can be proven with very simple mathematics. Just because something is published in a 'standard' doesn't mean it's true or even valuable.

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