Question about Process Capability Index


A component has a specification of 12 cm with a permitted tolerance of .015 cm and the process that produces it has an inherent standard deviation of .002 cm. Which of the following is true?

a. The Process Capability Index is 2.5

b. This is an example of "Gold Plating"

c. The Voice of the Process exceeds the Voice of the Customer

d. All are true


According the mock test of my PMP class, the right answer is d... I agree with answers b and c, but I cannot find the process capability index 2.5... I don't remember about this index during class, I tried some formulas found in the Internet but it is like something is missing. Hopefully someone here feels comfortable to help me, that would be greatly appreciate, thanks!

 Its CL is 0.015

it means 

+/-3sigma = +/- 0.015

hence SD = 0.005

where as process SD is 0.002

P C Index =required SD/ process SD

= .005/.002 = 2.5



Thank you very much! I am still confused about how you get SD = 0.005... Is it simply 0.015 divided by 3?


As per pmbok generally CLs are +/-3sigma.

but in any specified case it may vary.

 Thank you again. I knew about this page in PMBOK, but I was confused during the mock test, because there is no clear formula to learn about this kind of question; I mean I know by heart all the PMP formula about EVM, forecasting, project selection, procurement, PERT and SD, etc and etc, but there is no clear formula about Process Capibility Index... 

 Yes you are right.

it is 1st time we saw such question.

like many other terms not mentioned in pmbok or so far good authors could not include them, this is like that.


crushPMP's picture

Don't see Process Capability Index in PMBOK, is this in the syllabus? where is this question from?

 I have not found anything about Process Capability Index in the PMBOK. It is a question from the more test provided by PMP exam prep class I had in Feb 2013.

Can anyone explain how this an example of Gold Plating? Is it because the process is providing better performance than expected by the customer?

Gold plating is something additional that you're providing when customer has not asked for it. PMI doesn't approve of gold plating.

Since when has Process Capability Index crept into the PMP exam content?

I don't think it was there when I did my PMP at least (PMBOK IV).

~ Diba


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