Ppk vs Cpk - A Good, clear explanation and How Mini-Tab Handles Certain Statistics

Thanks for the explanation. That makes more sense than just about anything else I've heard -- I guess because it's from a user's standpoint. (And I did read it -- how about that!)

Also see:
https://Elsmar.com/ubb/Forum10/HTML/000028.html

To Ppk or Cpk, that is the question.

Let me see if I can get to the skinny of things in 50 words or less.

Cpk – The capability index for a stable process. The estimate of sigma is based on within subgroup variation. Cpk can only be calculated when the process is stable.

Ppk – The performance index. The estimate of sigma is based on total variation. Ppk is to be calculated if less than 100 samples or when the process is chronically unstable but meeting the specifications and in a predictable pattern.

… well unfortunately that was 67 words. But as you see there is a difference in the definition between the two. Now you will know if some body reports a process index using the Cpk calculation, you know that the process is stable and mature. And if you receive a Ppk calculation, you know that the process is either in it’s infant stages or chronically unstable but meeting the specifications and has a predictable pattern.

Did this clear or muddy up the water of your understanding of the difference between Cpk / Ppk.

You say that one can assume that someone reporting Ppk is indicating one thing and someone reporting Cpk is indicating something else. I doubt it, in most cases, because there is so much confusion or variation of interpretation (as witnessed by the replys above). But what you say makes sense.

I agree by reading the above posts that there appears to be some level of confusion. However, the definition I have provided comes straight from the beautiful blue books we have all grown to love, the QS manuals. In my opinion, there is very little “wiggle room” on its interpretation.

Section I.2.2.9.2 of the PPAP Third Edition clears up this whole Cpk – Ppk thing.

Hello!

Is there any formulas to count Percent Nonconforming from the CPK? The PDF says to find Z-Score curve area in the standard normal table but I just have an Excel chart where I would like to have Percent Nonconforming values calculated automatically in the basis of CPK.

Jari Maatta:

Just my opinion, but I would be wary using a Cpk value to determine % nonconforming. Cpk values can be based upon too many variables such as sample size, frequency, etc... The "Z" table assumes a perfect situaton.

There is a general guide in the AIAG FMEA manual (page 39) and what is the PDF?

ASD...

Which metrics to use?

The point of my earlier post was that ALL of them can and should be used - IF you understand the differences.

If you have someone who doesn't understand their meaning, use the Pp & Ppk (where the estimate of the sigma is the simple sample standard deviation of all the data glommed together - this is EXACTLY what the pre-AIAG Cp & Cpk was.) These reflect the actual performance of the process.

How to calculate %-nonconforming?

My advice is to calculate the mean and sample standard deviation and then use the z-statistic to calculate the probablitiy of observing a variate outside the spec limits.

Using Excel notation:

%-nonconformance = =100*(NORMDIST(LSL,Mean,StdDv,TRUE)+(1-NORMDIST(USL,Mean,StdDv,TRUE)))

For upper/lower bound just remove the relavent portion.

Good morning from Finland again.

Ken K, your formula seems to work very fine! I checked the results with Minitab too and it seems to give ~ same results for percent non-conforming! And as PDF, I meant Don Winton's "Process Capability Studies" document. But thanks again, I'll ask you again if I have some problems with my calculations..

Oh, I didn't know you are using MINITAB. By all means, they'll give you the correct % nonconformance - assuming you've entered the spec limits in correctly. I've had some people check the Hard Limit boxes by mistake. MINITAB treats these as a lower/upper bound (they even label it that way on the chart), and assumes that the process CANNOT go beyond that point, therefore it treats this as a one-sided spec and doesn't calculate %-nonconforming beyond those bounds.

By the way, how's the weather in Finland?? It has been kind of warm (30C+) and rainy in Chicago the last few days. More so than usual. Good for the crops though.