Cpk Help needed - Explain to me where I made a mistake

S

silvestri

#1
Hi all,
I taught to be a SPC expert but my vanity has been punished by the Gods.

When you find, in a capability sample, even one measurement outside the tolerance range, at least the Cpk must be < 1 ?!? Right???

It's incredible, I have found a sample where one item is smaller than the lower tolerance limit and Cpk 1,79 !!

It's the 3rd time I rewrite all the formulas and result is always the same.
Could someone take a look to the attached worksheet and explane me where I mistake?

Thanks.
 

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E

Eric York - 2010

#2
You calculations are correct. Why do you say Cpk must be less than 1 if one measurement is outside your spec limits? In your example the process is well centered and has extremely low variability with the exception of the one outlier.

Remember, from an intuitive perspective Cpk is a measure of 'centering' and 'spread' of your overall process with respect to your spec limits. Look at the calculation of Cpk. What goes into it? xbar, s, and your spec limits. Nowhere do you see mention of individual measurements. Sure, one outlier can have a big effect on the mean. But in this case you have 49 measurements almost exactly in the middle of your spec range, and only one outlier.

Here's an exercise: look at the difference in mean, std dev, and Cpk if you eliminate that outlier!

Eric
 
G

gszekely

#4
Re: Bloody Cpk! Help needed

Hello Silvestri !
I'm not an SPC guru, but where from you got the info, that if 1 point is out of tolerance, your capability index should be less than 1 ?
I put your data in my excell sheet, got the same results as you got.
I put nominal dimension 1,8 and subgroups of 5.

I think that there will be some more inputs.
BR
Gy
 
E

Eric York - 2010

#5
Of course you are correct! The difference between Ppk and Cpk is that in Ppk one uses the sample standard deviation instead of the overall process sigma, and that Ppk can be misleading if s is not truly representaive of sigma. I've become complacent ...
 
S

silvestri

#6
Thank you very much to all.

I must surrender to the evidence that one point outside specs limits doesn't cause capability indexes to be < 1. (I am so glad my formulas still work):eek:

But, if you are interested in this thread, what would you do if you were in this situation: would you validate the starting of production of this process or not?

Thank you again for your help
 
D

dendy

#7
But, if you are interested in this thread, what would you do if you were in this situation: would you validate the starting of production of this process or not?
Based on the data gathered, there was probably a measurement error or a deviation in raw material. The process is valid but I would have a second look at the nc part. Let me guess: this is probably a stamping operation?
 
Y

Yew Jin

#8
Hi guys,

Below is my 20cents.

It is very important to ensure our data is in the normal distribution before we conduct the process capability study.

By using the Anderson Darling normality test on the data, the data is not in normal distribution.

We are based on the assumption that the data represent a sample from a normally distributed population to study the Capability indexes. Thus, it is necessary to correct for nonnormality in the data when using the normal capability analysis procedures. If the observations come from a skewed distribution, the capability indexes such as ppk and cpk may look "good", yet many units will still be "out of spec".
 
S

silvestri

#9
Thanks Yew for this suggestion. Following your suggestion, I have just downloaded an excel sheet for a normality test. I think I will implement in our Cp-Cpk figures sheet.

Yes Dendy, it's a process very near to stamping. This variable and the sample are referred to crimping of terminals on electric wires.
 
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