Ppk vs Cpk: Big Differences - What conclusion can I reach about my process?

R

ricardo.qre

#1
After using the data from our process we got:
cpk=2.42
ppk=0.76

we used a I-MR chart to get the data.

I understand ppk should be used as long term and cpk short term, since the differences on how the sigma is calculated, but

What conclusion can I reach about my process?
I mean, by having ppk vs cpk big differences is there any typical conclusion we can obtaine from this?

and what about if cpk = ppk (or very close)

waht does that mean ?
 
Elsmar Forum Sponsor

Miner

Forum Moderator
Staff member
Admin
#2
Cp and Cpk are calculated using a standard deviation that reflects short term variation that ideally represents only common cause variation. For sub-grouped data, it is calculated using only the variation within subgroups. For I-MR data it is calculated using the variation between successive measurements.

Pp and Ppk are calculated using a standard deviation that reflects long term variation that represents the inclusion of any special cause variation that might exist in the data. This is calculated using an overall variation rather than the within sub-group or between successive data.

See this Minitab Technical Support Document for the actual equations for these standard deviations, and this Technical Support Document for the calculation of Pp and Ppk.

Based on a Cpk of 2.42 and a Ppk of 0.76, you process has little common cause variation, but a lot of special cause variation. If you were to eliminate the special cause variation, your process would be highly capable, but under the current conditions the process is incapable.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
You can draw even better conclusions if you plot your data in time series and look at it.

You probably have a lot of small shifts or slow trends/cycles.

If you post your data we can provide better advice.
And tell us something about the process
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
Also, having some idea of the process is important to answer this question. You can answer the question "what is the difference between Cpk and Ppk?" without that, but you cannot answer why they are different. It can go to a larger question, such as: are these indices correct for your process variation distribution? That is why time series data taken with an adequate measurement system is critical.
 
Thread starter Similar threads Forum Replies Date
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
Sean Kelley 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
Barahir 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
A How to interpret Minitab Results? What is difference in Cpk and Ppk values? Using Minitab Software 2
J Cpk & Ppk - Reviewing a PPAP Package - Some questions Statistical Analysis Tools, Techniques and SPC 8
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. 15
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
S Calculating Cpk/Ppk Simultaneously in SPC software Capability, Accuracy and Stability - Processes, Machines, etc. 6
J The Formulas of Ppk and Cpk in VDA 4.1 Capability, Accuracy and Stability - Processes, Machines, etc. 14
Similar threads


















































Top Bottom