Calculating Cpk when sample size equals to 1

M

M.Yung

#1
Hi all.

I have been given the task of compiling documentation required for a PPAP submission. One of these documents require me to calculate the Cpk of the process involved. However, only 1 product can be made per every 8 minutes. I could be wrong, but this leads me to believe that my sample size would be 1 (n=1)

From my understanding, I believe I need to take least 2 samples at same time in order to calculate the Cpk (e.g. 5 samples at time 10:30pm).

How would I go about calculating the Cpk for this?

In addition, there are three Specification Limits that need measuring. Does this mean that 3Cpk's are needed?

Cheers
 
Elsmar Forum Sponsor

Bev D

Heretical Statistician
Staff member
Super Moderator
#2
yes you need a cpk value for each specification (characteristic)

Cpk can be calculated with n=1 as long as you have multiple samples of n=1.

There is also nothing preventing you from take n=3 or n=5 sequential parts (time between parts doesn't matter)

The key is that Cpk is for an extended period of time not a small period of time. So a daily Cpk or a Cpk for every lot is not valuable (nor is it what a PPAP is looking for)
 
M

M.Yung

#3
Cpk can be calculated with n=1 as long as you have multiple samples of n=1.

There is also nothing preventing you from take n=3 or n=5 sequential parts (time between parts doesn't matter)
So I'm just imagining the scenario in my head...I would take 1 part at 10:08, another part at 10:16, and another part at 10:24. Would this make the sample size 3 (n=3) if I was combine them together?

Cheers
 

Darius

Quite Involved in Discussions
#6
So I'm just imagining the scenario in my head...I would take 1 part at 10:08, another part at 10:16, and another part at 10:24. Would this make the sample size 3 (n=3) if I was combine them together?

Cheers
:caution:You can, but there must be a rational of the sample size election, check for rational sampling on the net, if there is no theoretical reasons to take 3 is just gibberish, just play with numbers.
 
M

M.Yung

#8
Hi all

Can anyone recommend a book that they personally found very helpful on Cpk's? I realise its writing style may not be what I'm use to, but I'd still be willing to have a look.

Cheers
 
Last edited by a moderator:

reynald

Quite Involved in Discussions
#10
Steve,
I have been meaning to watch your Red Bead videos since 2 years ago and I'm glad I was able to do so now.
Impressive on how you and the president handled it.

I learned a lot as well.

Thanks again for sharing.
 
Thread starter Similar threads Forum Replies Date
T Calculating LCL, UCL, Cp, and Cpk in an Excel Spreadsheet Statistical Analysis Tools, Techniques and SPC 3
D Calculating Cpk on Non-Normal Data Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 10
B Please share a template for calculating Cp Cpk Document Control Systems, Procedures, Forms and Templates 3
M Calculating Cp and Cpk - Is my calculation correct? Capability, Accuracy and Stability - Processes, Machines, etc. 7
M Calculating correct Cpk - Getting different answers Capability, Accuracy and Stability - Processes, Machines, etc. 3
K Calculating StdDev and Cpk on Multiple Populations Capability, Accuracy and Stability - Processes, Machines, etc. 2
D Calculating Cpk for Tubing Wall Thickness - Extrusion Process - How to approach? Statistical Analysis Tools, Techniques and SPC 16
D Use of an outlier in calculating Cpk/Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 8
S Calculating Cpk/Ppk Simultaneously in SPC software Capability, Accuracy and Stability - Processes, Machines, etc. 6
L Calculating Cpk on a single sided (unilateral) tolerance Capability, Accuracy and Stability - Processes, Machines, etc. 22
C Calculating Cp and Cpk for Product with only LSL (Lower Specification Limit) Capability, Accuracy and Stability - Processes, Machines, etc. 28
L Calculating the RFT using the Cpk value Capability, Accuracy and Stability - Processes, Machines, etc. 3
A Calculating Cp and Cpk on a Non-Normal Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 14
Marc Calculating Defect Rate (PPM) from a Cpk value Capability, Accuracy and Stability - Processes, Machines, etc. 8
M Cp & Cpk - How are you calculating the standard deviation? Capability, Accuracy and Stability - Processes, Machines, etc. 3
C Calculating your KPIs ISO 13485:2016 - Medical Device Quality Management Systems 7
K Calculating Heat Dissipation Manufacturing and Related Processes 1
Q Calculating Defect Rates ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 4
S Formula for Calculating NoGo Major Diameter for UN gages Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
P Calculating a weight for a machine for the CE label CE Marking (Conformité Européene) / CB Scheme 0
R Remote Support - Calculating Number of Employees IATF 16949 - Automotive Quality Systems Standard 2
G Calculating Tolerance of a Panel Meter with a 2 degrees of an Arc General Measurement Device and Calibration Topics 0
V Product Development - When to start calculating Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 3
J Calculating Reliability for Subsystems in Series Reliability Analysis - Predictions, Testing and Standards 15
Stijloor Calculating (3rd Party) Audit Days for Company with Seasonal Employees General Auditing Discussions 3
T Formulas for Calculating Coefficients for RTD / PRTD Calibration According to ITS-90 General Measurement Device and Calibration Topics 5
J Calculating part variation from historic data for GRR study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
P Purpose of calculating Ta, Tb & R Square in Linearity Study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
P Purpose of calculating Uncertainty value in calibration study Measurement Uncertainty (MU) 5
M Calculating MTBF for plug-in PCI cards Reliability Analysis - Predictions, Testing and Standards 2
J Calculating impact force, expressed in lbs, of a 5 lb weight dropped 48 inches Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
K Calculating Capability of a process when data is skewed in the upper range Capability, Accuracy and Stability - Processes, Machines, etc. 4
A Calculating Accuracy for a Multimeter General Measurement Device and Calibration Topics 1
M Calculating Coefficients for an RTD (Resistance Temperature Detector) Probe General Measurement Device and Calibration Topics 3
T Calculating Plating/Coating Weight for IMDS RoHS, REACH, ELV, IMDS and Restricted Substances 3
D Calculating Lower and Upper 3 Sigma Control Limits Statistical Analysis Tools, Techniques and SPC 2
M Calculating Adequate Receiving Inspection Sample Size Statistical Analysis Tools, Techniques and SPC 2
M Calculating Capability of Delivery Performance Capability, Accuracy and Stability - Processes, Machines, etc. 5
K Advice on Calculating Control Chart Control Limits Statistical Analysis Tools, Techniques and SPC 13
B Calculating Combined DPMO and Sigma Level for Two or More Different Work Areas Six Sigma 3
G Calculating the Fraction Defective Confidence Interval for a Lot Statistical Analysis Tools, Techniques and SPC 2
A Calculating Combined Measurement Uncertainty - VDA 5 Measurement Uncertainty (MU) 1
B Calculating Precision to Tolerance Ratio Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 9
D DPPM - Calculating the Defect Rate of a Software Test Statistical Analysis Tools, Techniques and SPC 9
L FMEA - Calculating RPN = S*O*D FMEA and Control Plans 3
I Shewhart Constants vs Central Limit Theorem in calculating Control Limits Statistical Analysis Tools, Techniques and SPC 18
S Is a Stable Process (within Control Limits) required for Calculating Pp, Ppk? Capability, Accuracy and Stability - Processes, Machines, etc. 6
M Calculating the IATF TS 16949 Recertification Audit Man Days - Clarification ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
J IEC60825 - Laser Safety - Calculating Maximum Permissible Exposure Correctly Other ISO and International Standards and European Regulations 1
P Calculating Process Capability from Data of attached spreadsheet Statistical Analysis Tools, Techniques and SPC 4
Similar threads


















































Top Bottom