# Individual & MR chart - Ppk or Cpk?

P

#### pagnonig

Dear All,

I've been through a lot of posts regarding Cpk and Ppk and the short/long term discussion on them.
Now, I would like to be sure I've catch the right practical meaning for both.

I'm collecting individual measurements to be plot in a I-MR chart.
having USL and LSL limits I would compute Ppk for this process using the Sigma (n-1) calculation and not the estimator R.avg/d2 because in this case I do not have subgroups.

Therefore I would say that Cpk is meaningless for Individual readings.

Anyway, I've noted that both the estimated and the calculated standard deviation tend to be the same value as the number of individual measurements becomes 50-60.

Thank you!
Giuseppe

#### bobdoering

Trusted Information Resource
It would be handy to know the process to ensure we understand the applicable distribution.

D

#### Duke Okes

Dear All,I've been through a lot of posts regarding Cpk and Ppk and the short/long term discussion on them.
Now, I would like to be sure I've catch the right practical meaning for both.
I'm collecting individual measurements to be plot in a I-MR chart.
having USL and LSL limits I would compute Ppk for this process using the Sigma (n-1) calculation and not the estimator R.avg/d2 because in this case I do not have subgroups.Therefore I would say that Cpk is meaningless for Individual readings.Anyway, I've noted that both the estimated and the calculated standard deviation tend to be the same value as the number of individual measurements becomes 50-60.Any comments on my approach?
Thank you!Giuseppe
Cpk vs. Ppk has more to do with whether all sources of variation have been included in the study, usually thought of as how many data points, or how long a time frame, have/has been used to do the calculation. It does not matter whether sigma comes from Individuals or Subgroups.

If you find that n-1 and R-bar/d2 give you the same number it probably just indicates that the process is in control.

P

#### pagnonig

It would be handy to know the process to ensure we understand the applicable distribution.
Normality test on available data passed successfully.
I'm collecting thickness measurement of a cable layer (like a tube wall).

#### bobdoering

Trusted Information Resource
Normality test on available data passed successfully.
I'm collecting thickness measurement of a cable layer (like a tube wall).
If I had to choose, I would use the sigma of the individuals - hoping that my frequency was sufficient to catch the bulk of the variation. As a curious note, how did you arrive at your data acquisition frequency?

But, as Duke Okes indicated,the difference should not make an overwhelming impact on the final analysis. I would not lose sleep over either one.

#### Miner

##### Forum Moderator
Staff member
Normality test on available data passed successfully.
I'm collecting thickness measurement of a cable layer (like a tube wall).
Is this an extrusion or drawing process? If it is and you have not already done so, I recommend performing an analysis for autocorrelation. If autocorrelation exists, the period between measurements should be longer than the period of autocorrelation.

Measurements within the period of autocorrelation are essentially pure measurement error.

P

#### pagnonig

Is this an extrusion or drawing process? If it is and you have not already done so, I recommend performing an analysis for autocorrelation. If autocorrelation exists, the period between measurements should be longer than the period of autocorrelation.

Measurements within the period of autocorrelation are essentially pure measurement error.
Thank you everyone for your support!

This is an extrusion process.
measurements are taken at the start of each produced reel (2000 meters).

I think that measurements are so distant between each other that autocorrelation should be a low risk.

anyway, how can I perform such analysis? Coudl you give me any reference on this?
thank you again!

D

#### Darius

I'm collecting individual measurements to be plot in a I-MR chart.
having USL and LSL limits I would compute Ppk for this process using the Sigma (n-1) calculation and not the estimator R.avg/d2 because in this case I do not have subgroups.

Therefore I would say that Cpk is meaningless for Individual readings.
Giuseppe
Cpk is not meaningless, in the case of Individual and moving range the estimate of variation used to construct control limits is the same as the one's used to calculate cpk.

SIGMA_ESTIMATE =MR_average/d2 = MR_average/1.128

UCL = MEAN+ 3* SIGMA_ESTIMATE = MEAN + 3/1.128 * MR_AVERAGE= MEAN+ 2.66* MR_AVERAGE

LCL = MEAN- 3* SIGMA_ESTIMATE = MEAN - 3/1.128 * MR_AVERAGE= MEAN- 2.66* MR_AVERAGE

CP= (USL - LSL)/(6*SIGMA_ESTIMATE)
CPK = MIN(MEAN-LSL, USL-MEAN)/(3*SIGMA_ESTIMATE)

Last edited by a moderator:
T Implementing SPC - Control Chart for Individual Measurements Statistical Analysis Tools, Techniques and SPC 9
B Color Individual Points on a Control Chart? Using Minitab Software 7
K Individual x Chart - Why use the equation is barX+2.66 bar(MR) Statistical Analysis Tools, Techniques and SPC 6
D Introducing Individual Chart to Accountants Statistical Analysis Tools, Techniques and SPC 23
A Individual/Moving Range Chart (IMR Chart) in Excel Excel .xls Spreadsheet Templates and Tools 8
D What is the minimum number of values to develop individual chart? Statistical Analysis Tools, Techniques and SPC 14
E Formulas and Constants - Control Limits for Individual and Moving Range Control Chart Statistical Analysis Tools, Techniques and SPC 2
M Individual or Moving Range Chart? Parts run in lots of 7 with 4 key wall thicknesses Statistical Analysis Tools, Techniques and SPC 5
A Cpk for Individual Chart Capability, Accuracy and Stability - Processes, Machines, etc. 1
Records Control - Does each individual record need to be numbered? Records and Data - Quality, Legal and Other Evidence 2
EU Shelf life claims for individual materials EU Medical Device Regulations 2
P Large data sets of continuous individual data - Estimated or actual deviation Capability, Accuracy and Stability - Processes, Machines, etc. 2
Can an individual apply for an MDEL in Canada, as an establishment? Canada Medical Device Regulations 6
G How should we document individual Support Processes ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 1
N Process Capability Study - Aggregate vs. Individual Processes Statistical Analysis Tools, Techniques and SPC 24
G Labeling requirements for individual units for medical devices 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 2
N Injection Molding - Median individual control charts Statistical Analysis Tools, Techniques and SPC 6
J Individual part traceability - ink/paint sprayer? IATF 16949 - Automotive Quality Systems Standard 4
Tracking response time with individual label I.D. Statistical Analysis Tools, Techniques and SPC 6
B Cleanroom Control Limits: Cumulative or individual? Statistical Analysis Tools, Techniques and SPC 11
I Dual Acceptance Criteria: Cpk and Individual Values Statistical Analysis Tools, Techniques and SPC 3
Mix Multi-Site/Individual Certification - Has this ever happened to anyone? Miscellaneous Environmental Standards and EMS Related Discussions 14
M Individual Assembly Routing Question Manufacturing and Related Processes 4
1 VBA Needed to Protect Individual Excel Cells Excel .xls Spreadsheet Templates and Tools 2
K Equipment Lifetime: Put in operation manual or a individual document/declaration Quality Manager and Management Related Issues 1
9001 Corporate Quality Audit - Yearly Audit on the Individual Business Units General Auditing Discussions 4
N Capability on Parallel Processes such as Individual Mould Cavities Philosophy, Gurus, Innovation and Evolution 22
P Responsibilities of Individual named as Submitter of 510(k) 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 4
P 510K Help for Individual Medical Device Inventor 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 5
K If an individual requests my ISO 9001 QMS Documents, do I have to supply it? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 7
B IMR or Individual Run SPC Charts In Minitab Using Minitab Software 3
Dr Wheeler weighs in on "Individual Charts Done Right and Wrong" Statistical Analysis Tools, Techniques and SPC 2
Technical File composition for a kit that consists of a number of individual products EU Medical Device Regulations 4
T Specification Conversion from Sets to Individual Measurements? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
Y Establishing and Measuring Individual Performance to Product Quality Standards Misc. Quality Assurance and Business Systems Related Topics 5
S How to measure Energy Use for individual users in a shared business Miscellaneous Environmental Standards and EMS Related Discussions 23
S Configuration Management compliance - Adding information to individual procedures AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 8
R Ford Individual Attribute R&R Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
S Authorization Individual Form - Competency to use certain equipment ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 11
N Including individual Quality Objectives in the Quality Manual? Document Control Systems, Procedures, Forms and Templates 6
I Resource Outsourcing Problems - Individual Performance Metrics - Any suggestions? Quality Tools, Improvement and Analysis 14
A Language Requirement for Individual EU & EFTA countries ISO 13485:2016 - Medical Device Quality Management Systems 2
M System Level Root Causes for each individual CAR generated in the system? Problem Solving, Root Cause Fault and Failure Analysis 10
M Control Plan Requirements - Should I identify all the individual drawing dimensions? FMEA and Control Plans 6
Monitoring Individual Quality Performance - Who is responsible? Career and Occupation Discussions 10
F Continuous Improvement Process Map - As an individual process? Process Maps, Process Mapping and Turtle Diagrams 5
J Looking for Individual Health Insurance options Career and Occupation Discussions 5
F Method for aggregating individual RPNs and determining an overall risk rating? FMEA and Control Plans 3
D Validating software in materials testing - Integrated system of individual components General Measurement Device and Calibration Topics 3
J Calculating Series System Reliability and Reliability for Each Individual Component Reliability Analysis - Predictions, Testing and Standards 4