Elsmar Cove Forum Header Graphic The Elsmar Cove Wiki More Free Files The Elsmar Cove Forums Discussion Thread Index Post Attachments Listing Failure Modes Services and Solutions to Problems Elsmar cove Forums Main Page Elsmar Cove Home Page
Miner's MSA (Measurement Systems Analysis) Blog 
Go Back   The Elsmar Cove Forum > Common Quality Assurance Processes and Tools > Capability, Accuracy and Stability - Processes, Machines, etc.
Forum Username

Wooden Line

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

Wooden Line
Search the Elsmar Cove
Search Elsmar
Monitor the Elsmar Forum
Follow Marc & Elsmar
Elsmar Cove Forum RSS Feed  Marc Smith's Google+ Page  Marc Smith's Linked In Page   Marc Smith's Elsmar Cove YouTube Page  Marc Smith's Facebook Page  Elsmar Cove Twitter Feed
Elsmar Cove Groups
Elsmar Cove Google+ Group  Elsmar Cove LinkedIn Group  Elsmar Cove Facebook Group
Donate and $ Contributor Forum Access
Courtesy Quick Links

Links that Elsmar Cove visitors will find useful in your quest for knowledge:

Howard's
International Quality Services
Marcelo Antunes'
SQR Consulting
Bob Doering's
Correct SPC - Precision Machining

NIST's Engineering Statistics Handbook
IRCA - International Register of Certified Auditors
SAE - Society of Automotive Engineers
Quality Digest Portal
IEST - Institute of Environmental Sciences and Technology
ASQ - American Society for Quality

Related Topic Tags
process capability, samples and sampling plans, cpk (related to process capability)
Reply
 
Thread Tools Search this Thread Rate Thread Content Display Modes
  #1  
Old 21st April 2012, 02:12 AM
maghsoodi

 
 
Posts: 16
Question Cpk - How to calculate when the number of samples is less than 30 pieces?

How is calculated When the number of samples is less than 30 CPK ?
Excel software to do this work?

Sponsored Links
  #2  
Old 21st April 2012, 04:02 AM
harrysons's Avatar
harrysons

 
 
Posts: 19
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.
Sponsored Links

  #3  
Old 21st April 2012, 05:18 AM
maghsoodi

 
 
Posts: 16
Re: Cpk - How to calculate when the number of samples is less than 30?

Quote:
In Reply to Parent Post by harrysons View Post

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
  #4  
Old 21st April 2012, 05:24 AM
maghsoodi

 
 
Posts: 16
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
  #5  
Old 21st April 2012, 06:49 AM
harry

 
 
Posts: 6,277
Re: Cpk

Quote:
In Reply to Parent Post by maghsoodi View Post

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.
  #6  
Old 21st April 2012, 11:15 AM
Jim Wynne's Avatar
Jim Wynne

 
 
Posts: 14,077
Re: Cpk - How to calculate when the number of samples is less than 30 pieces?

Quote:
In Reply to Parent Post by maghsoodi View Post

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.
Thanks to Jim Wynne for your informative Post and/or Attachment!
  #7  
Old 22nd April 2012, 11:25 AM
Sam Lazzara's Avatar
Sam Lazzara

 
 
Posts: 212
Re: Cpk - How to calculate when the number of samples is less than 30 pieces?

Here is some info on Cpk.
http://www.itl.nist.gov/div898/handb...ion1/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.
Thanks to Sam Lazzara for your informative Post and/or Attachment!
  #8  
Old 27th April 2012, 09:02 AM
Darius's Avatar
Darius

 
 
Posts: 538
Re: Cpk - How to calculate when the number of samples is less than 30 pieces?

Sorry for being a little late, 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.
Reply

Lower Navigation Bar
Go Back   The Elsmar Cove Forum > Common Quality Assurance Processes and Tools > Capability, Accuracy and Stability - Processes, Machines, etc.

Do you find this discussion thread helpful and informational?


Bookmarks


Visitors Currently Viewing this Thread: 1 (0 Registered Visitors (Members) and 1 Unregistered Guest Visitors)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Forum Search
Display Modes Rate Thread Content
Rate Thread Content:

Forum Posting Settings
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Similar Discussion Threads
Discussion Thread Title Thread Starter Forum Replies Last Post or Poll Vote
why CPK <1.0 regarding two pieces of control charts of torque wrench? Terry Statistical Analysis Tools, Techniques and SPC 2 30th November 2010 11:21 AM
Methods used to calculate Cpk value - Need help for Cpk Calculation jerry_Malaysia Statistical Analysis Tools, Techniques and SPC 8 25th September 2009 04:26 PM
How to compare Cpk and ppm (bad pieces) drebernik Capability, Accuracy and Stability - Processes, Machines, etc. 3 9th May 2007 09:46 PM
How much does Cpk > 1.33 represent in precentage of good pieces? tattva Capability, Accuracy and Stability - Processes, Machines, etc. 17 14th November 2003 10:09 AM
Can I calculate Cpk from Cp and Cr? dragonmaster Capability, Accuracy and Stability - Processes, Machines, etc. 5 3rd May 2000 08:38 AM



The time now is 12:44 AM. All times are GMT -4.
Your time zone can be changed in your UserCP --> Options.


   


Marc Timothy Smith - Elsmar.com
8466 LeSourdsville-West Chester Road, Olde West Chester, Ohio 45069-1929
513 341-6272
NOTE: This forum uses "cookies".