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

A

Andrews

#1
Capability: Short Term or Long Term?

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?
 
Elsmar Forum Sponsor
N

noboxwine

#2
My Two Cents

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 ! :smokin:
 

Atul Khandekar

Quite Involved in Discussions
#3
My Two Cents too

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

Thaumaturge
Super Moderator
#4
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.
 

Wes Bucey

Quite Involved in Discussions
#6
Statistics - Blessing and Curse!

Marc said:
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/library/capability/capability-menu.htm
 
Last edited:
C

CastaDave

#7
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
 
B

ben sortin

#9
Just make sure you understand the sources of variability in the surrogate process.

If you have a capability index of one and only one person looks at it one time then one report will probably do until one addresses management commitment.
 

Mike S.

Happy to be Alive
Trusted Information Resource
#10
QeSmithy said:
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
- For a Six Sigma process, Cp = 2.0 and Cpk = 1.5

I Hope this helps!

Smithy
Welcome, Smithy.

I thought a Cpk of > 2 was required for a "six sigma" process. Can you double-check?

Here is another opinion on the debate, FWIW, from an article I have by Dr. Mehernosh Kapadia:
____________
In 1991, ASQ / AIAG task force published the "Statistical Process Control" reference manual, which presented the calculations for capability indices ( Cp, Cpk ) as well as process performance indices ( Pp, Ppk ).

The difference between the two indices is the way the process standard deviation ( s ) is calculated.

Cpk uses s which is estimated using ( R-Bar / d2 ) or ( S-Bar / C2 ) .

Ppk uses the calculated standard deviation from individual data where s is calculated by the formula :


So the next question is which metric is best to report Cpk or Ppk ?
In other words, which standard deviation to use - estimated or calculated ?
Although both indices show similar information, they have slightly different uses.

Ppk attempts to answer the question "does my current production sample meet specification ?" Process performance indices should only be used when statistical control cannot be evaluated.

On the other hand, Cpk attempts to answer the question "does my process in the long run meet specification?" Process capability evaluation can only be done after the process is brought into statistical control. The reason is simple: Cpk is a prediction, and one can only predict something that is stable.
____________
Dr. Mehernosh Kapadia is General Manager - Quality Engineering, Supply Chain Management Business Group at Tata AutoComp Systems Limited.

He has a Master's Degree in Mechanical Engineering from Penn State University, USA and MBA from NTU, USA. He is a Six-Sigma Master Black Belt and an ASQ certified CQE & CRE. He obtained his Doctorate from The University of Bombay
 
Thread starter Similar threads Forum Replies Date
B Short term vs Long term Capability - We ship millions of parts in a year Capability, Accuracy and Stability - Processes, Machines, etc. 7
kedarg6500 Sample Size for Short Term & Long Term Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 2
Chennaiite Interpretation of Short term vs. Long term Capability Studies Statistical Analysis Tools, Techniques and SPC 3
J Process Capability Sheet for a short term 30 piece study needed Statistical Analysis Tools, Techniques and SPC 1
V Machine short term capability study with Cmk index in automotive industry Capability, Accuracy and Stability - Processes, Machines, etc. 3
D Cpk vs. Ppk - Long Term vs. Short Term - Capability vs. Stability Capability, Accuracy and Stability - Processes, Machines, etc. 29
T Process Capability - Long-term vs. Short-term Statistical Analysis Tools, Techniques and SPC 9
A Short Run Capability - Batch quantity is 15,000 to 20,000 Statistical Analysis Tools, Techniques and SPC 5
S Precontrol and Process Capability - CNC Lathes that run small batches - Short Runs Statistical Analysis Tools, Techniques and SPC 12
Q Capability - CPk comparison values Capability, Accuracy and Stability - Processes, Machines, etc. 12
R Capability analysis - What is going on this chart? Manufacturing and Related Processes 15
H Capability Data for Paint Thickness on Painted Parts Statistical Analysis Tools, Techniques and SPC 10
N Is capability applicable for a destructive test? Capability, Accuracy and Stability - Processes, Machines, etc. 9
S Capability or Gage R&R Study for Leak Tester? Reliability Analysis - Predictions, Testing and Standards 15
D Pre-Production Capability Assesment Reliability Analysis - Predictions, Testing and Standards 5
T Iec 60601 Impedance and current-carrying capability IEC 60601 - Medical Electrical Equipment Safety Standards Series 1
S Machine Setup for a CNC Machine Capability Study Capability, Accuracy and Stability - Processes, Machines, etc. 1
D O Ring capability and measurement - What is the automotive 'norm' for capability studies on O Rings? General Measurement Device and Calibration Topics 3
M Measuring Capability of Process with Multiple Specifications Capability, Accuracy and Stability - Processes, Machines, etc. 6
G Capability Study for Tapped Hole Reliability Analysis - Predictions, Testing and Standards 3
E Sampling for capability studies for variables and attributes Capability, Accuracy and Stability - Processes, Machines, etc. 4
L How to evaluate the process capability of a data set that is non-normal (cannot be transformed and does not fit any known distribution)? Capability, Accuracy and Stability - Processes, Machines, etc. 12
T Final process capability results - What I am supposed to present? Cp and CpK? APQP and PPAP 11
G Plotting capability study data - I have a bimodular data distribution Capability, Accuracy and Stability - Processes, Machines, etc. 4
A Statistic Tools to Know Capability of Sealing Machine for Seal Width Capability, Accuracy and Stability - Processes, Machines, etc. 3
Q How to perform Process Capability for true position Statistical Analysis Tools, Techniques and SPC 10
V Generic requirements regarding capability study in automotive Capability, Accuracy and Stability - Processes, Machines, etc. 1
A Capability Analysis for Packaging Seal Strength with spec. >0.1 Kgf using Minitab Using Minitab Software 6
J Initial capability for injection molding part Capability, Accuracy and Stability - Processes, Machines, etc. 6
Ron Rompen Ford Method - Position Capability with MMC Modifier Capability, Accuracy and Stability - Processes, Machines, etc. 14
O Capability or Gage R&R Study for Leak Tester Reliability Analysis - Predictions, Testing and Standards 11
S Understanding control chart and measurement capability Statistical Analysis Tools, Techniques and SPC 2
L IATF 16949 Cl. 8.3.3.2 Manufacturing process design input - Process capability IATF 16949 - Automotive Quality Systems Standard 1
M IATF 16949 - Capability study on non critical dimensions? Statistical Analysis Tools, Techniques and SPC 4
J Are Capability Requirement(s) Valid for All Characteristics? Capability, Accuracy and Stability - Processes, Machines, etc. 6
Y Process Capability for Non-Normal Data - Philosophical Questions Capability, Accuracy and Stability - Processes, Machines, etc. 6
M Process Capability in a High Precision Environment Capability, Accuracy and Stability - Processes, Machines, etc. 7
M CAAC-145 Manuals - Looking for examples of MOM's, MMM's Capability Lists, etc. Federal Aviation Administration (FAA) Standards and Requirements 13
J Powdercoating Capability Employing a Robotic Arm Manufacturing and Related Processes 1
L Excel Template for Cgk Study wanted (Capability Gauges Study) Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 12
V Product Development - When to start calculating Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 3
V Capability for Correlated Charteristics Capability, Accuracy and Stability - Processes, Machines, etc. 2
S Proving Process Capability of a Packing Machine Capability, Accuracy and Stability - Processes, Machines, etc. 6
R Visual Attribute Inspector Capability and KAPPA Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
ncwalker Process Capability Study - Aggregate vs. Individual Processes Statistical Analysis Tools, Techniques and SPC 24
KyloDen Capability Study and Qualifying a Supplier Capability, Accuracy and Stability - Processes, Machines, etc. 5
V Process Capability for parameters with non-normal distribution. Capability, Accuracy and Stability - Processes, Machines, etc. 16
J Process Capability - How much percentage of tolerance? VDA Standards - Germany's Automotive Standards 3
bobdoering Unilateral Tolerance Capability Calculations Capability, Accuracy and Stability - Processes, Machines, etc. 1
bobdoering Unilateral Tolerance Capability Calculation Imported Legacy Blogs 0

Similar threads

Top Bottom