Ppk or Cpk - Preliminary Process Capability vs. Process Performance

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#11
Tim Folkerts said:
Of course, both extremes are sometimes useful, and both extremes sometimes obscure interesting information. The challenge is to find a reasonable degree of simplification for the need at hand. Sometimes a single number, like Cpk or mu, is helpful. Sometimes it is almost valueless.

(The challenge of choosing the right index and performing the correct calculations is problematic, but it is a separate issue. )


Tim F
OK, I agree that you probably can't do with the indeces. But choosing the right index and performing the correct calculations is not sufficient for gaining the proper information. If you never look at the control chart to do if the data are in control, or worse, don't know how to read a control chart or interpret what it means, you will still get the wrong answer.

People are required to have driver's licenses to drive a car, it is amazing that people figure they don't need a license to work with data. :whip:
 
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Tim Folkerts

Super Moderator
#12
Steve Prevette said:
People are required to have driver's licenses to drive a car, it is amazing that people figure they don't need a license to work with data. :whip:
:agree1: :)

Perhaps I should change my signature to
Tim Folkerts: 007, license to calculate. :lmao:
 

niotusen

Registered Visitor
#14
Thanks for the answers!
Back to my problem, have I understand it right:
- That my customer is asking for wrong indexes? They call it Cpk when it should be Ppk?
/Niotusen
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#15
niotusen said:
Thanks for the answers!
Back to my problem, have I understand it right:
- That my customer is asking for wrong indexes? They call it Cpk when it should be Ppk?
/Niotusen
they probably are. ask them what they want the data for, which formula to use...the answer to the reason for the data should tell you which to give them. I always default to the Ppk value taken over time (depending on how my process varies so that I have a representative sample) as it provides the most useful information (given that indexes are fairly value weak) for teh actual long term perfomance of a stable system.
Cpk can understate the full range of variation if the subgroups aren't rational
 

niotusen

Registered Visitor
#16
Yehhaa!
Now the problem begin?
My customer does not know how to calculate Ppk or Cpk? What is the different they say? Isn´t this great?

To the next question that com up:

What are the rules of Ppk then 1.67 or??
What should we follow on our processes?

Ppk over 1,67 or Cpk over 1,33?

Or must the both be under control at the sametime we produce??

My customer has no qlue and I ´m confusius? (Thanks this forum is to help)

Thanks!
 

The Taz!

Quite Involved in Discussions
#17
Try to look at it this way. . .

A sprinter runs on a short groomed track. . . a cross country runner runs on country roads or streets. The Sprinter can average 14 MPH on his track. But put the Sprinter on country roads, and he may average 11 MPH.

The Sprinter has the potential to run 14 MPH. . . but with the variation experienced on the country roads (hills, ruts, turns, etc.), he would only be expected to average 11 MPH.

Ppk is Short Term Process Potential - Sprinter - Minimal Variation - 14 MPH - 1.67 Ppk minimum

Cpk is Long Term Process Capability - Long Distance - More Variation - 11 MPH - 1.33Cpk Minimum

I hope this analogy helps. This is the way I describe the difference. Same process. . . just looking at it under difference circumstances.
 
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