Capability: Short Term or Long Term? Cpk= Long term and Ppk= Short term?

[Andrews]

I had the opportunity to attend a training programme conducted by one of our customer where the lecturer / facilitator told us something that was in contradiction to what I learnt earlier.

They said that Cp,Cpk is related to short term capability and Pp,Ppk is related to long term capability.What do the forum users feel about it?

[noboxwine]

You'll typically find, in Automotive:

Cpk= Long term---due to estimated std dev
Ppk= Short term---due to calculated std dev

And, MHO, if the long-term process is stable, this is the way to go. If not a stable process, Cpk is always worthless.

In Software, I have typically seen just the opposite.

I don't see, whether or not it's car parts or CD's, where an estimated sigma (Cpk) is good for any short term capability.

Though, very interested in other opinions as always.

Hope this helps ! Have a day !

[Atul Khandekar]

I agree with noboxwine.

Stability is a prerequisite for Process Capability. This can be established using a control chart. A long term study would imply that the process has had a chance to run thorugh all possible cycles and therefore all the common and special causes of variation have been covered. In such cases Cpk is calculated using sigma=Rbar/d2.

Process Performance Ppk involves short span runs of may be 100 consecutive parts. Some of the special causes of variation may not have ocurred at all.

Here are two earlier threads on the same topic:
http://Elsmar.com/Forums/showthread.php?t=1994
http://Elsmar.com/Forums/showthread.php?t=4048

[howste]

Now I'm confused. I don't have a copy of the AIAG SPC manual here, so I pulled out Ford's CSRs because I remember it being talked about in there. But they contradict themselves! In 4.37 it says...

The choice of the capability index used for initial process studies - Cpk (predictive), or Ppk (historical) – shall be based solely on the nature of the process data collected

But in Table B it says...

After process stability has been demonstrated and capability has been calculated, the most recent point on the control chart and the historical process capability indices (Cpk/Cp) may be used to determine appropriate actions.

It sounds like Ford is confused also. My understanding has always been as stated above - that Cpk was historical and Ppk was short-term.

[Marc]

So the answer is???

[Wes Bucey]

Originally Posted by Marc

So the answer is???

ASQ's Body of Knowledge for the Quality Manager certification test describes
Cpk as long-term, stable (Cpk > 1.3 = capable)
and
Ppk [Potential Process Capability] as short-term, stability undetermined (Ppk > 1.67 = capable) The difference in ratio of being acceptable to be called "capable" is just a fudge factor.

The difference in what is capable ratio is that because the run is short, Ppk does not include the normal variability that will be seen in a full production run.

Now ask me, "What do I really believe?"

I really believe that reliance on such ratios for short runs (Ppk) is really not as informative as pure control charts.

There are links to interesting articles on the topic at
http://www.qualityadvisor.com/librar...ility-menu.htm

[QeSmithy]

I've stumbled across this debate on this forum, which is a coincidence because I was having the same arguement with one of my Quality Engineers.

He also states that his training told him that Cp/Cpk related to Long term, and Pp/Ppk related to short term.

I am a certified Six Sigma Blackbelt, and my training pushed me in the other direction. After a heated debate, I reviewed the training files from the Six Sigma Institute which states the following:

When evaluating process capability…
- A short term study is conducted to see how good a process can be. Data is collected over a short period of time during which the process is influenced only by random causes.Cp, Cpk
- A long term study is conducted to see the process’ actual long-term performance. Data is collected over a long enough period of time such that the process is influenced by both random and non-random causes.Pp, Ppk
- The Process Sigma Level is the z value corresponding to a short term study (centered distribution, when using variable data).
- The Short Term Process Sigma Level can be estimated from a long term study by the adjustment: zST = zLT + 1.5.
- The 1.5 Sigma adjustment accounts for the shift and drift of the short term distribution over time.
- When variable data is used, the capability indices Pp and Ppk are computed:
Cp indicates the process’ potential capability if it were perfectly centered between specification limits.
Cpk takes into account off-centering.
- For a Six Sigma process, Cp = 2.0 and Cpk = 1.5
- When attribute data is used in a long term study, the Long Term Process sigma Level is the z value corresponding to the proportion defective or DPO. The Short Term Process Sigma Level (reported) = zST = zLT + 1.5.


I Hope this helps!

Smithy

[Atul Khandekar]

Smithy,
Thanks and Welcome to the Cove.