The Elsmar Cove Business Standards Discussion Forums More Free Files Forum Discussion Thread Post Attachments Listing Elsmar Cove Discussion Forums Main Page
Welcome to what was The Original Cayman Cove Forums!
This thread is carried over and continued in the Current Elsmar Cove Forums

Search the Elsmar Cove!

Wooden Line
This is a "Frozen" Legacy Forum.
Most links on this page do NOT work.
Discussions since 2001 are HERE

Owl Line
The New Elsmar Cove Forums   The New Elsmar Cove Forums
  Statistical Techniques and 6 Sigma
  Statistically Valid Sample Sizes For SPC

Post New Topic  Post A Reply
profile | register | preferences | faq | search

UBBFriend: Email This Page to Someone! next newest topic | next oldest topic
Author Topic:   Statistically Valid Sample Sizes For SPC
J. Otto
posted 27 September 2000 05:59 PM           Edit/Delete Message   Reply w/Quote
Hello All,

I am looking for some input into sample size considerations and frequecy of sample pulls for production lots that we are calculating Ppk values on. Currently we are pulling 4 parts every 2 hours for each process to calculate Ppk values on. My concern is that this is the 4 parts every 2 hours is regardless of cycle time or production lot size.

Is there an issue with being statistically valid in our calculations based on sample size and frequency. As an example, I may have two seperate production lots that I am going to run for 24 hours. The first process may have a cycle time of 10 seconds and the second process may have a cycle time of 20 seconds. After 24 hours of production, this would result in first process having twice as much product than the second process. Should we be increasing our sample size and/or frequency of pulls for the first process???

If there is any information available or if anyone has any suggestions on what makes for statistically valid sample sizes and frequency of sampling, please let me know.

Any help will be greatly appreciated.



IP: Logged

Rick Goodson
Forum Wizard

Posts: 102
From:Wuakesha, Wisconsin, USA
Registered: Aug 2000

posted 28 September 2000 12:39 PM     Click Here to See the Profile for Rick Goodson   Click Here to Email Rick Goodson     Edit/Delete Message   Reply w/Quote
Your question is very good, but unfortunately complex and not easily answered in a paragraph or two. Never the less, some thoughts and suggestions.

From a protection point of view, absolute sample size of a random sample is more important than its relative size compared to lot size. This is readily apparent if you look at Operating Characteristic Curves for a constant sample size and with differing lot sizes.

With regard to sample size in X-bar and R charts, the essential idea is to select subgroups to minimize the opprotunity for variation with-in a subgroup. We want them small. However, a size of four works best because with a sample of four the distribution of X-bar is nearly normal even though the samples are taken from a non-normal universe.

The frequency of the sampling is an economic decision based on what happens to material produced between samples if we find an out of control condition. Theoretically frequency should be often when analyzing a process initially and the reduced once the process is under control and running well.

Take a look at the following references for detail on sample size selection and frequency:

Statistical Process Control by Grant and Leavenworth, ISBN 0-07-844354-7

AT&T Statistical Quality Control Hanbook (Mine is so old id does not have an ISBN)

Quality Control by Dale Besterfield
ISBN 0-13-025668-4

IP: Logged

All times are Eastern Standard Time (USA)

next newest topic | next oldest topic

Administrative Options: Close Topic | Archive/Move | Delete Topic
Post New Topic  Post A Reply Hop to:

Contact Us | The Elsmar Cove Home Page

Your Input Into These Forums Is Appreciated! Thanks!

Main Site Search
Y'All Come Back Now, Ya Hear?
Powered by FreeBSD!Made With A Mac!Powered by Apache!