SBS - The Best Value in QMS software

Cpk & Ppk - Reviewing a PPAP Package - Some questions

J

johnny xu

#1
Recently my colleagues ask some questions about CPK &PPK when reviewing PPAP package.
It added to my confusion.
Who could tell me correct answer.
1. Which is long-term capability index or short-term as for CPK or PPK.
2. why defined CPK is at least 1.33 and PPK is at least 1.67?
3. is CPK value always less than PPK value when implementing SPC for calculating CPK & PPK on the same spreadsheet?
4. when do we calculate CPK or PPK during preparing PPAP package?

THX
:thanx:
 
Elsmar Forum Sponsor

qusys

Trusted Information Resource
#2
Recently my colleagues ask some questions about CPK &PPK when reviewing PPAP package.
It added to my confusion.
Who could tell me correct answer.
1. Which is long-term capability index or short-term as for CPK or PPK.
2. why defined CPK is at least 1.33 and PPK is at least 1.67?
3. is CPK value always less than PPK value when implementing SPC for calculating CPK & PPK on the same spreadsheet?
4. when do we calculate CPK or PPK during preparing PPAP package?

THX
:thanx:
Hi,
if you are an automotive supplier CPK shall be at 1.67. You shall agree with your customer which is the index for your process capability.
Cpk/Ppk should be calculated in the initial capability studies.:bigwave:
 

Jim Wynne

Staff member
Admin
#3
Recently my colleagues ask some questions about CPK &PPK when reviewing PPAP package.
It added to my confusion.
Who could tell me correct answer.
1. Which is long-term capability index or short-term as for CPK or PPK.
The long-term and short-term designations make no sense and only cause confusion, but the standard definitions are that Cpk = long term and Ppk = short term. You can find more about in this discussion thread: Capability: Short Term or Long Term
2. why defined CPK is at least 1.33 and PPK is at least 1.67?
I don't know where that information might have come from. The 4th Edition PPAP manual, on pages 8 and 9, says that (a) you should agree with the customer on the index and criteria to use and (b) that as default values, 1.67 or greater is considered acceptable; a result of 1.33 to 1.66 inclusive "may be acceptable" and results less than 1.33 "...currently does not meet acceptance criteria." If you have a customer with different criteria, you should ask them for their rationale if you're curious.
3. is CPK value always less than PPK value when implementing SPC for calculating CPK & PPK on the same spreadsheet?
See this discussion thread: Cpk and Ppk-Within Subgroup Variation is Always Smaller than Total Variation?

4. when do we calculate CPK or PPK during preparing PPAP package?
When data is available from the control charting that happened during the "significant production run." Before calculating either value, you should ensure that the process is statistically stable, but the PPAP manuall calls for Ppk calculation "...for processes with known and predictable special causes and output meeting specifications..."
 
D

Darius

#4
1. Which is long-term capability index or short-term as for CPK or PPK.
There is too much counfusion in such terms, I prefere within and overall variation estimates, as I readed:
ppk, cpk es short-term because uses within variation estimate and ppk uses overall variation estimate (overall = within + between), but as Jim said it can be taken the other way

2. why defined CPK is at least 1.33 and PPK is at least 1.67?
IMHO, to take in account unseen variation (that will happen the sooner or latter but not in the current data),:bonk:funny I think (look like Z shift <none sence>) but as qusys said, "the value must be determined by your customer"

3. is CPK value always less than PPK value when implementing SPC for calculating CPK & PPK on the same spreadsheet?
:yes:Wheeler said that it can be the other way if incorrect sampling is done, but as far as I know, cpk can be equal to ppk (if no between variation exists whithin variation = overall variation) or smaller

4. when do we calculate CPK or PPK during preparing PPAP package?
The experts said: "use cpk if the process is stable", others said that there is no process stable to perfection even on simulators. in advanced topics on SPC wheeler said not to calculate indicators if the process is not stable. ppk is recommended for initial studies.:bonk:
IMHO, use something as somebody said "to manage, you have to measure", and ppk and cpk are just that, maybe not perfect but a way to measure the process behavior. Maybe your customer will tell wich one do he like, but question him about wich variation estimate uses, some use ppk and show it as cpk:nope:
 
Last edited by a moderator:

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#5
And don't forget to check to see if the capability calculation is even applicable. In the PPAP 4th ed: 2.2.11.5 Processes With One-Sided Specifications or Non-Normal Distributions NOTE specifically states: "The above mentioned acceptance criteria (2,2,11,3) assume normality and a two-sided specification (target in the middle). When it is not true, using this analysis may result in unreliable information....." After that they throw in some murky gibberish that essentially means they have no concrete idea what to do in such a case.

Fact is, if people are just calculating these indices and do not even look at the distribution, they can either be worse off, overcontrolling, or just wrong. Any Cpk would be accompanied by the distribution and evidence of an acceptable Gage R&R (less than 10%). Otherwise, the "normal" distribution may just be gage and measurement error, and the true distribution may not be applicable.
 
S

Sturmkind

#6
Excellent comment, Bob! Some features like stamping holes will almost never be Cpk=>1.33 because they are run to the high tolerance limit for tool wear and to take advantage of Maximum Material Condition modifiers on positional tolerances.

The Cpk=1.33 and Ppk = 1.67 came from the AIAG 3rd edition PPAP manuals. Fortunately, the folks at AIAG recognized that this was statistical 'terrorism' for the publication of the 4th edition.

The original definition of Cpk and Ppk was Cpk was more than 125 sub-groups and Ppk was less than 125 sub-groups. The mathematical change to use population standard deviation in the denominator happened between the 2nd & 3rd edition of the AIAG SPC manual.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#7
I still think the capability numbers are pretty much an abomination of statistics. What single number would describe the variation your life has experienced? Oh....and it should also represent any variation you will experience in the future. That's pretty much what they are trying to describe with these indices.
 
Thread starter Similar threads Forum Replies Date
C Capability in sewing processes CP,CPk,PP,PPk Capability, Accuracy and Stability - Processes, Machines, etc. 4
W LTPD, AQL, Ppk and Cpk validation sampling plan table Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
A Interpretation with regards to Ppk > Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 14
E SPC Production - Getting Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 8
V When to use Cp Cpk and Pp Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 4
S Value of Ppk or Cpk when targeting thinner material Statistical Analysis Tools, Techniques and SPC 4
B Cpk vs Ppk to look at the Capability by Fixture Capability, Accuracy and Stability - Processes, Machines, etc. 4
V Customer requires CpK 2 and PpK 1.5 Capability, Accuracy and Stability - Processes, Machines, etc. 2
R Which of the following indicators is important for Operations - Ppk, Cpk, or Cpm? Statistical Analysis Tools, Techniques and SPC 15
U Cpk and Ppk for different Countries/Standards Capability, Accuracy and Stability - Processes, Machines, etc. 7
B Understanding why my CpK and PpK are low, and LCL Statistical Analysis Tools, Techniques and SPC 20
S Which one is better to calculate, Cpk or Ppk? Statistical Analysis Tools, Techniques and SPC 11
A How to calculate Cpk & Ppk if the tolerance changes during the period? Capability, Accuracy and Stability - Processes, Machines, etc. 5
B Formula Data Validation Set for Cp, Cpk, Pp & Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 1
F Please share an MS Excel sheet to calculate Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 1
R Ppk vs Cpk: Big Differences - What conclusion can I reach about my process? Six Sigma 3
A How to interpret Minitab Results? What is difference in Cpk and Ppk values? Using Minitab Software 2
S Ppk & Cpk - Within Subgroup Variation is always smaller than Total Variation Capability, Accuracy and Stability - Processes, Machines, etc. 18
V CNC Process Validation Cpk Ppk & In-Process Inspection Inspection, Prints (Drawings), Testing, Sampling and Related Topics 7
S Pp/Ppk & Cp/Cpk - Which is appropriate for Long/Short Term Study? Statistical Analysis Tools, Techniques and SPC 20
R Cpk & Ppk and Differences between Minitab and an Excel Spreadsheet Results Statistical Analysis Tools, Techniques and SPC 4
S How is Cpk /Ppk value calculated in Minitab since I have subgroup size of 1 Capability, Accuracy and Stability - Processes, Machines, etc. 1
Q When to use Cpk or Ppk for a Process Capability Study? Capability, Accuracy and Stability - Processes, Machines, etc. 14
A Deciding on when to go for Cp & Cpk or Pp & Ppk Study? Statistical Analysis Tools, Techniques and SPC 3
C Understanding the relationship of Pp, Ppk, Cp, Cpk Six Sigma 4
S Cpk and Ppk Differences and Questions Capability, Accuracy and Stability - Processes, Machines, etc. 5
E Cpk and Ppk template wanted Capability, Accuracy and Stability - Processes, Machines, etc. 3
Q ASQ CSSBB Question on Cpk and PPk , can you solve it ? Capability, Accuracy and Stability - Processes, Machines, etc. 15
M Calculation of Cpk and Ppk for an unstable process Statistical Analysis Tools, Techniques and SPC 7
Q PPK, CPK of >= 1.67 Automotive Requirement and Auditing IATF 16949 - Automotive Quality Systems Standard 1
P Individual & MR chart - Ppk or Cpk? Capability, Accuracy and Stability - Processes, Machines, etc. 9
K Normality of distribution prior to Cpk and PpK Statistical Analysis Tools, Techniques and SPC 4
R Can Ppk value be greater than Cpk and if yes, under what conditions? Statistical Analysis Tools, Techniques and SPC 10
C Is it possible to get Pp,Ppk more than the within subgroup (Cp,Cpk's) indices? Statistical Analysis Tools, Techniques and SPC 7
S Are Pp and Ppk more useful than Cp and Cpk as a benchmark for further studies? Statistical Analysis Tools, Techniques and SPC 1
J Using Pin Gages as a Measurement Tool for Ppk/Cpk studies Capability, Accuracy and Stability - Processes, Machines, etc. 11
B Difference between Ppk (Preliminary), Cpk, Performance Capability Statistical Analysis Tools, Techniques and SPC 2
D Real life example of the difference between Cpk and Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 0
N Cpk vs Ppk - Which should you use and How to Calculate Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 25
V Quality Control Index - We thought that Cpk or Ppk would be good choices Capability, Accuracy and Stability - Processes, Machines, etc. 6
Tim Folkerts Interpreting the common capability indices: Cp, Cpk, Pk, and Ppk. The Reading Room 5
J Machine issue in Cpk/Ppk and Control Plan - PPAP Submissions APQP and PPAP 5
D Use of an outlier in calculating Cpk/Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 8
N Analysis of non-normal stratified data for cpk/ppk? Rupture test Capability, Accuracy and Stability - Processes, Machines, etc. 6
F Ppk/Cpk value with 1-sided specification - Only given a lower specification Statistical Analysis Tools, Techniques and SPC 1
S Ppk/Cpk for geometrical true position of hole with MMC Capability, Accuracy and Stability - Processes, Machines, etc. 17
Howard Atkins Validation of Cpk and Ppk calculations - Sheet to check Cpk and Ppk values Statistical Analysis Tools, Techniques and SPC 4
S What is the meaning of "k" in Cpk and Ppk? And Cpk vs. Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 15
A Capability Indexes for ISO/TS16949 - Cpk & Ppk after PPAP Statistical Analysis Tools, Techniques and SPC 6
C I need help verifying formula's in a database for LCL UCL Cp Cpk PPk Statistical Analysis Tools, Techniques and SPC 14

Similar threads

Top Bottom