# Calculation of σ within subgroups for Cpk in Minitab

J

#### John Koh

In Minitab, the std deviation used to calculate Cpk is the sigma for within subgroups whereas for Ppk, the overall sigma is used to calculate. My question is that since there are so many subgroups (for example 25 subgroups), which subgroups' sigma will be used to compute the final Cpk?

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
Most statistical software use the most common definition of Cpk adn Ppk:
Cpk is for short term capability and uses the within subgroup variation. it gives you an idea of the potential Capability if you were to eliminate the subgroup to subgroup (assignable cause) variation*.
Ppk is for long term capability and uses the total standard deviation - both within and between - and gives you an idea of the actual Performance.

in general use, many people simply refer to Cpk as the total performance capability (the Ppk formula in most software). by the way, this is the ORIGINAL definition of Cpk, before someone dreamed up the Ppk thing.

When you have a homogenous process - or have rationally subgrouped - Cpk and Ppk will converge to the same value.

you are probably looking for the Ppk Minitab output...

*this distinction is actually in direct opposition to the 'requirement' that processes be statistically in control before calculating Cpk/Ppk...but hey the whole Cpk/Ppk thing is statistically invalid so what the heck...

S Within Part Variation - Excel .xls calculation spreadsheet Excel .xls Spreadsheet Templates and Tools 11
IATF 16949 News Risked Based Audit Day Calculation IATF 16949 - Automotive Quality Systems Standard 2
Uncertainty calculation for an average Measurement Uncertainty (MU) 1
Scrap % calculation for a year. Manufacturing and Related Processes 3
Question for: Cg & Cgk calculation General Measurement Device and Calibration Topics 3
21 CFR 211.103 Calculation of Yield Pharmaceuticals (21 CFR Part 210, 21 CFR Part 211 and related Regulations) 5
Cost of Poor Quality (CoPQ) - Scrap element of calculation includes lost margin? Misc. Quality Assurance and Business Systems Related Topics 7
Analysis variation among the lots during cpk calculation. Design and Development of Products and Processes 1
PPM calculation of customer complaints for PMS Customer Complaints 10
Overvoltages consideration in Applied Part for RMS Calculation. IEC 60601 - Medical Electrical Equipment Safety Standards Series 6
Cpk Calculation - I analyse a double seam cans Manufacturing and Related Processes 15
Lifetime Support Calculation - Thermal Operating Profile Reliability Analysis - Predictions, Testing and Standards 7
Stock Reorder point calculation Lean in Manufacturing and Service Industries 1
3 Operator Std. Dev. and uncertainty calculation Measurement Uncertainty (MU) 2
F Determining what type of scrap to include in my internal PPM calculation Quality Manager and Management Related Issues 5
Holes position inspection - Calculation of centre distance and plug sizes of a gauge Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
P Any official doc/spec on different types of Yield calculation? Manufacturing and Related Processes 0
M Benchmarking Process Data - Man-day calculation gone wrong Benchmarking 5
H Taguchi mixed model DoE [L16 (4^3 2^6): factors interaction and ANOVA calculation Using Minitab Software 3
P-Chart Control Limit Calculation help Statistical Analysis Tools, Techniques and SPC 1
Sample Size Calculation (Confidence Interval and Reliability) - Medical Devices Reliability Analysis - Predictions, Testing and Standards 9
E Questions regarding Cpk Calculation - Should we be using LSL/USL or LCL/UCL? Capability, Accuracy and Stability - Processes, Machines, etc. 5
Number of Distinct Categories" or NDC Calculation in MSA Studies Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 8
Sample Size Calculation for Image Analysis - Microscopy Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
Rule 5.2h - Site Separation - Audit Manday Calculation (IATF 16949) IATF 16949 - Automotive Quality Systems Standard 17
Gage R&R - Is my sheet calculation wrong? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 21
Practical use of Heat Input calculation for Manual MIG Welding Manufacturing and Related Processes 4
J CMM Uncertainty Calculation Question - Deviations Measurement Uncertainty (MU) 9
S Scrap % Calculation Formula Manufacturing and Related Processes 10
Reliability calculation for repairable system Reliability Analysis - Predictions, Testing and Standards 14
F Calculation of reduction on audit time - IAF MD1:2007, IAF MD5:2015 and IAF MD11:2013 General Auditing Discussions 1
Is there a calculation in Net Promoter Score (NPS) ? Statistical Analysis Tools, Techniques and SPC 3
Unilateral Tolerance Capability Calculation Imported Legacy Blogs 0
M Help with uncertainty calculation in the calibration of a Pyrogen Test Processor Measurement Uncertainty (MU) 7
H ROI (Return on Investment) Calculation of Employees ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 7
PPM - Filtering Defects out of calculation Quality Tools, Improvement and Analysis 5
E How does Non-Linear UUT affect TAR (Test Accuracy Ratio) Calculation? Measurement Uncertainty (MU) 3
O Takt Time Calculation for Long Runner Product Lean in Manufacturing and Service Industries 5
M Packaging Material Requirement Calculation Sheet for a Bottle Packaging Line Manufacturing and Related Processes 3
B Histogram beginner dilemma - Manual Calculation vs. JMP 7 Quality Tools, Improvement and Analysis 12
S SPC Control Limit Calculation Statistical Analysis Tools, Techniques and SPC 2
N Procedure/calculation for determining the foot pressure of dial thickness gauge General Measurement Device and Calibration Topics 11
T Calculation Question - Calculate the runner weight for a part Manufacturing and Related Processes 2
A Accuracy Calculation of Leakage Current for an Electrical Safety Analyzer General Measurement Device and Calibration Topics 5
E Composite Position Calculation on a 12 X 4 Hole Pattern Inspection, Prints (Drawings), Testing, Sampling and Related Topics 13
M Sigma Calculation in Excel Capability, Accuracy and Stability - Processes, Machines, etc. 3
H Uncertainty Measurement Calculation in Chemical Testing Measurement Uncertainty (MU) 1
V How to use a Calibration Certificate in Uncertainty Calculation for a Test Measure Measurement Uncertainty (MU) 13
D Siemens MES software DPMO Calculation Six Sigma 4
N Uncertainty calculation for a voltage measurement Measurement Uncertainty (MU) 3