Cpk - How to calculate when the number of samples is less than 30 pieces?

Elsmar Forum Sponsor

harrysons

Realistic Quality
#2
Re: Cpk - How to calculate when the number of samples is less than 30?

If you talk in a sample (units) size may be some one expert can support you :)
IMHO the CPK is pertinent by subgroup size, Sub group Frequency and the last is Number of supgroup and Generally the number of subgroups is 25 or more..to get the good test for stability,

Excel software to do this work?
excel able to this job.
 

maghsoodi

Starting to get Involved
#3
Re: Cpk - How to calculate when the number of samples is less than 30?

If you talk in a sample (units) size may be some one expert can support you :)
IMHO the CPK is pertinent by subgroup size, Sub group Frequency and the last is Number of supgroup and Generally the number of subgroups is 25 or more..to get the good test for stability,

Excel software to do this work?
excel able to this job.
stability,

My number for the CPK is 20 pieces. Not twenty-subgroup
 

maghsoodi

Starting to get Involved
#4
Cpk

How is calculated CPK When the number of samples is less than 30 Pieces ?(My number for the CPK is 20 pieces. Not twenty-subgroup)
THANKS BABAK MAGHSOODIAN
 

harry

Super Moderator
#5
Re: Cpk

How is calculated CPK When the number of samples is less than 30 Pieces ?(My number for the CPK is 20 pieces. Not twenty-subgroup)
THANKS BABAK MAGHSOODIAN
Welcome to the Cove.

There is no need for a new thread as the original thread title had been edited to clearly reflect your situation - less than 30 pieces.

Weekends are typically slow so please be patient. The crowd will be back by Monday morning, US time.
 

Jim Wynne

Staff member
Admin
#6
How is calculated When the number of samples is less than 30 CPK ?
Excel software to do this work?
You can't derive a legitimate Cpk number from a sample of 20 pieces or 30 pieces or 100 pieces, for that matter.

Cpk assumes:

  • A statistically stable process
  • Rational subgrouping and chronological sampling while production is running
  • A large enough number of measured subgroups to represent the expected output of the process over time.
Cpk is not a very meaningful index under the best of circumstances, but unless the conditions above are fulfilled it's pretty much a waste of time.
 

Sam Lazzara

Trusted Information Resource
#7
Here is some info on Cpk.
http://www.itl.nist.gov/div898/handbook/pmc/section1/pmc16.htm

Having a stable process is a prerequisite and you cannot determine that from a single sample. Another important consideration is making sure your distribution is "normal" and your sample standard deviation (s) is a good estimate for the population standard deviation (sigma). Do not assume your distribution is normal/gaussian unless you test it, and a sample of 20 does not provide great power for that test.

Also note what is stated on the NIST link:
Most capability indices estimates are valid only if the sample size used is 'large enough'. Large enough is generally thought to be about 50 independent data values.

The math is the easy part.
 
D

Darius

#8
Sorry for being a little late:deadhorse:, have been bussy.... but this other tread is good for the matter.

Sample Size Does Matter

On that post, I try to say, you can calculate with almost any sample size but, the important thing is What is the worst case value for such estimate. Maybe many of us like to show good numbers, but being real is important, and most of us take the estimates as the true value, not just an estimate.

There are other issues far more important than sample size, as is stated on this tread, the normality is the real deal, and sample size does not assure the Gaussian Distribution.
 
Thread starter Similar threads Forum Replies Date
B How to calculate CPK with n = 1 Manufacturing and Related Processes 2
A Simple 125 piece part Capabilty Excel .xls sheet to calculate Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 3
S Which one is better to calculate, Cpk or Ppk? Statistical Analysis Tools, Techniques and SPC 11
G Should I calculate both Cp and Cpk for a Unilateral Tolerance? Statistical Analysis Tools, Techniques and SPC 8
A How to calculate Cpk & Ppk if the tolerance changes during the period? Capability, Accuracy and Stability - Processes, Machines, etc. 5
F Please share an MS Excel sheet to calculate Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 1
D Use of Customer Specifications to calculate Cpk IATF 16949 - Automotive Quality Systems Standard 13
B How to calculate Cpk on Perpendicularity (Cyl Tol Zone at MMC) Capability, Accuracy and Stability - Processes, Machines, etc. 32
F How to Calculate Cpk with only a Minimum Tolerance Capability, Accuracy and Stability - Processes, Machines, etc. 11
C How to Calculate and Generate a Cpk Chart Statistical Analysis Tools, Techniques and SPC 4
B If I know x-bar, spec limits, and Cpk, can I calculate the control limits? Capability, Accuracy and Stability - Processes, Machines, etc. 16
J Methods used to calculate Cpk value - Need help for Cpk Calculation Statistical Analysis Tools, Techniques and SPC 8
H How can I calculate Cp & Cpk and make a control chart for maintenance activities? Capability, Accuracy and Stability - Processes, Machines, etc. 3
N Cpk vs Ppk - Which should you use and How to Calculate Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 25
A Sampling Plans and Cpk - Ways to Calculate the Sample Quantity Inspection, Prints (Drawings), Testing, Sampling and Related Topics 15
N Capability study - What Values do I have to input to calculate Cp and Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 9
B How do I calculate Cpk with a single Control Limit? Capability, Accuracy and Stability - Processes, Machines, etc. 15
S Cmk and Cpk - When will we need to calculate Cmk? Machine Capability Capability, Accuracy and Stability - Processes, Machines, etc. 4
J How do I calculate the percentage of errors, when I know min, max, mean, cp and CPK Capability, Accuracy and Stability - Processes, Machines, etc. 3
D Can I calculate Cpk from Cp and Cr? Capability, Accuracy and Stability - Processes, Machines, etc. 5
W LTPD, AQL, Ppk and Cpk validation sampling plan table Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
M Cp and Cpk for straightness and parallelism Capability, Accuracy and Stability - Processes, Machines, etc. 8
Q Capability - CPk comparison values Capability, Accuracy and Stability - Processes, Machines, etc. 12
D Tolerance definition based on expected Cp/cpk Reliability Analysis - Predictions, Testing and Standards 14
K Analysis variation among the lots during cpk calculation. Design and Development of Products and Processes 1
S Recommendation for user friendly Gaga R&R and Cpk software Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 10
A How to set up Continuous CpK monitoring of an injection mold process Reliability Analysis - Predictions, Testing and Standards 7
A Interpretation with regards to Ppk > Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 14
U Cpk Calculation - I analyse a double seam cans Manufacturing and Related Processes 15
Proud Liberal Cp / Cpk on position using multiple MMC bonuses Capability, Accuracy and Stability - Processes, Machines, etc. 2
I Can we use pin gauges to measure an accurate Cpk ? Capability, Accuracy and Stability - Processes, Machines, etc. 9
T Final process capability results - What I am supposed to present? Cp and CpK? APQP and PPAP 11
F CpK on a manufactured assembly? Reliability Analysis - Predictions, Testing and Standards 7
M How is the Cmk and Cpk calculated? Statistical Analysis Tools, Techniques and SPC 0
O Cpk for Unilateral Tolerance Capability, Accuracy and Stability - Processes, Machines, etc. 9
O CPK with a P value less than 0.005 Manufacturing and Related Processes 8
A Short Term vs. Long Term SPC Study - Where is Cp and Cpk Defined Statistical Analysis Tools, Techniques and SPC 8
A Cpk Formula seems off, need help!? Capability, Accuracy and Stability - Processes, Machines, etc. 11
D Cp/Cpk on Gages for MSA (vs. Cg/Cgk) Capability, Accuracy and Stability - Processes, Machines, etc. 21
E Questions regarding Cpk Calculation - Should we be using LSL/USL or LCL/UCL? Capability, Accuracy and Stability - Processes, Machines, etc. 5
B Maximum Material Condition with Bonus - Determine Cpk for this one kpc Measurement Uncertainty (MU) 2
Hami812 Cpk Setting Tolerance - Cart before the Horse? (Wifi Routers) Capability, Accuracy and Stability - Processes, Machines, etc. 3
T Calculating LCL, UCL, Cp, and Cpk in an Excel Spreadsheet Statistical Analysis Tools, Techniques and SPC 3
Proud Liberal Cpk on Position w/ MMC on Location and Size Capability, Accuracy and Stability - Processes, Machines, etc. 3
E SPC Production - Getting Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 8
S Track Cpk trend Statistical Analysis Tools, Techniques and SPC 5
M Cp & Cpk about a diameter measured with min.,max. and average values. Capability, Accuracy and Stability - Processes, Machines, etc. 13
P Non-normal Data Cpk Statistical Analysis Tools, Techniques and SPC 5
alonFAI Cpk for Solder Paste Height Statistical Analysis Tools, Techniques and SPC 5
I As a part of validation, should i include the acceptance criteria of CPk index >1.33? Qualification and Validation (including 21 CFR Part 11) 4

Similar threads

Top Bottom