Cpk vs. Ppk - Long Term vs. Short Term - Capability vs. Stability

R

Rob Nix

#21
Please read this entire thread and do a search of similar threads. Much has been written and debated.

Minitab's definition serves only to confuse the issue. However that stats used in the denominator are correct - Cpk uses an estimate of sigma, generally attained through control charts (which presupposes long term) and Ppk uses sigma (standard deviation) because the entire population is known (suggesting short term), and so the "performance" is known.

But, then again, don't worry about it. Either way, you get a good idea of how well you are staying within the spec tolerances, without being too nit-picky (assuming, as always, your process is in control).
 
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Jim Wynne

Staff member
Admin
#22
d_rock30 said:
Has anyone tried minitab software. It defines cpk as short term capability and ppk as long term capability. The difference simply lies on the formula. Cpk uses the D2 constant and Ppk uses the standard deviation.
That's almost correct; standard deviation comes in two varieties: Sample and Population. The former is used in calculating Cpk and the latter in Ppk calculations. The Greek lowercase sigma (s) is widely and incorrectly used in quality to represent both varieties, but sigma correctly identifies the population standard deviation while "s" is used to represent the sample standard deviation. Here's some more information:Wikipedia: Standard Deviation
 
K

KenK - 2009

#23
MINITAB 14's process capability terminology matches that of the new second edition of the AIAG SPC reference manual. MINITAB was ahead of the game there.

There is an odd problem though. In order to have MINITAB 14 match the formulas found in the AIAG SPC manual, you will need to do the following:

Starting with the MINITAB default settings, when using either the Capability Analysis or the Capability Sixpack, go to the Estimate options that effect the estimation of the standard devaition. The ONLY way to match the AIAG SPC manual completely is to:

1. Only use the Rbar method of estimating the standard deviation, and
2. Uncheck the "Use unbiasing constants" checkbox. This is NOT the default.

This will provide the correct within and overall standard deviations.

The within standard deviation based upon Rbar will use the biasing constants regardless choice (an biased Rbar estimate doesn't exist), which is what AIAG uses. The overall standard deviation will be biased. This is what you want. This is the sample standard deviation that you learned in school and the one AIAG uses.

Unfortunately you will have to uncheck the unbiasing constants checkbox everytime you use these tools. There is no preference setting in Options that will take care of this for you without messing up other stuff. I've told Minitab about this and I'm sure they'll correct it soon.

If you select the Sbar method, one of the two standard deviations will be incorrect, depending on whether the ubiasing constants checkbox is checked or not. If it is checked, then the overall standard deviation will not be correct. If the box is NOT checked, then the within standard deviation will not be correct. You can't win using Sbar. Minitab needs to fix this little oddity.

Leave the unbiasing constants option in Tools > Options checked - this is the default - or else many of the control chart calculations will be incorrect.
 
D

d_rock30

#24
does it mean that even AIAG will soon release its new edition and we will again have new understanding of the terminologies?
 
R

RMedrano

#25
KenK!

thanks for pointing out the problem with minitab.


We have a computerized SPC system here that automatically calculates those statistics, but I couldnt figure out why pasting the same info into Minitab would give me different results!


You Rock!

:biglaugh: :D
 
P

Piotr Stoklosa

#27
Hello,

The thread looks a little old but a topic is still fresh. There is still a lot of discussion on what is "short term capability" and what is "long term capability", especially when you try to link it with capability indices. The source of the problem are in my opinion different interpretations among OEM customers and also between statistical "schools".

I have made a little survey from this point of view and here are the results. In rows there are specific car producers, in columns you can find specific pairs of indices and terms used by the producers.



As you can see, there is a big mix-up with it. Especially, that German producers introduce additional Cm/Cmk indices and also name them "short term".

Because of these differences my suggestion is to stick with AIAG SPC Manual, which defines Pp/Ppk as ?process performance? and Cp/Cpk as ?process capability?. It helps to conduct discussion without long/short term notions and we can reserve ?long term? and ?short term? for the length of the observation period.

I am waiting for your comments and additional information about other OEM (and other tiers) terminology. What is especially interesting for me that I have found no one OEM customer that uses Six-Sigma naming convention (e.i. Cp/Cpk ? short term, Pp/Ppk ? long term). Do you know anyone?
 
S

Sean Kelley

#28
MiniTab defines Pp/Ppk as actual (overall) capability and Cp/Cpk as potential (within) capability.

Customers would generally want the Ppk which would showed all error introduced. Inside the Cpk would help in telling you how much room for improvement with the existing process there is. If they are close then there may be a need to change the process in ored to make improvements. If there is a large gap then the existing process can be improved on by analyszing where the error is introduced and reducing these sources.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#29
well the automtive world (AIAG) isn't the only industry out there...

actually in your table both GM and Ford have the correct naming convention from when Ppk was introduced. (in the beginning there was only Cp/Cpk and it didn't specify a standard deviation - the original assumed it was the total standard deviation and not the within subgroup standard deviation. In a homogenous stable process the within subgroup variation is a decent estimator of the total SD)

Cp stood for Process capability: Capability_process and Cpk stood for Process Capability in real life. the k was the offset of the mean from the nominal.

Then someone got cute and added Ppk to mean Process_Performance (long term) and Cpk to mean Process_Capability (short term) - now GM and Ford both refer to these descriptor 'correctly'.

The phrase Short term refers to the use of the withing subgroup variation , which in a classical Shewhart chart of a homogenous, stable process is very short term.

Long term refers to the use of the total standard deviation which can be a very long time - from subgroup to subgroup.

this was articulated directly by the Six Sigma folks at Motorola. GE and other companies also have referred to Cpk as short term and Ppk as long term.
Most statistical software uses this designation.

Then Ford got cuter and decided that neither adequately addressed the need for protype or preproduction analysis when there was only one lot, operator, equipment, raw material lot, and it was alimited build run. so they used the 'Ppk formula' to calculate the short term capability when there was no subgrouping.

The original article (unless someone knows of an earlier article?) introducing capability indices is: "Reducing Variability - A New Approach to Quality", Quality Progress, July 1984.


I also recommend reading the October 1992 issue of the Journal for Quality Technology devoted entirely - and reluctantly - to the topic. The intent was to deal with this 'fake' statistical index and kill it from serious consideration by serious Quality and statistical professionals. The January 2002 issue also contains some review of indices since the 1992 publication as well as an article and discussion regarding the Cpk/Ppk confusion and other problems with the indices.

And no one did it better than Bert Gunter in his 4 part column: The Use and Abuse of Cpk" in Quality Progress January, March, MAy and July 1989


Recommending what to call them is as futile as admonitions to just say no.
 
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P

Piotr Stoklosa

#30
Thank you for your comments. All you write about Cp/Cpk and Pp/Ppk is clear for me and I agree with it, but I do not want to start again a discussion which approach is better, because it was discussed many times here.

I just wanted to show the differences in terminology between automotive producers and resulting problems in communication with them.
 
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