Sampling Strategy for Process Control - How to develop a Sampling Plan

J

joker33

#1
hello;

this is my first thread in this forum and not even so sure if I did the correct statement for my question :)

I would like to learn more details about how to develop sampling plan;

* The purpose of sampling
* The Central Limit Theorem and why sampling works for process control
* How to define subgroup size
* Rational samples and stratification
* Identify a sampling plan’s major components
* Describe considerations for determining appropriate sample size and frequency


thanks in advance...
u.y.
 
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bobdoering

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#2
Re: Sampling Strategy for Process Control

..this is my first thread in this forum and not even so sure if I did the correct statement for my question :)

I would like to learn more details about how to develop sampling plan.
* The purpose of sampling

So you don't have to measure every part. Use statistics of the sample to estimate the behavior of the population.

* The Central Limit Theorem and why sampling works for process control

I usually don't rely on CLT to explain anything.

* How to define subgroup size

It depends on the process, its within and between variation.

* Rational samples and stratification

Depends on the process distribution.

* Identify a sampling plan’s major components

Measurement technique and frequency.

* Describe considerations for determining appropriate sample size and frequency

It depends on the process, its within and between variation and distribution.


The fact is, not to be too simplistic but rather accurate - the correct answer to all of these questions is "it depends". There is no equation or rubber stamp that can produce a more accurate sampling plan than actual process understanding - including sources of variation and error.

Do you have an actual process to prepare a sampling plan for, or is this an academic question?
 
J

joker33

#3
Re: Sampling Strategy for Process Control

thanks for clarification however yes it is academic and I will be glad to know more details of them
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#4
a good start is to purchase a good book on SPC and read it:
Grant and Leavenworth
Fair and Wise
Wheeler

then come back with specific questions.

you can also use the search feature here for each of the topics you list and see what we've written in the past
 
K

kabus - 2011

#5
Hi! This is also my first time doing this. i found it by chance browsing on the web to get some info related to sampling plans. I need to develop a procedure for sampling plans related to a packaging line that produces bottles and blisters. I am confused in terms of which shall I use: ANZI or the OC curve (alpha / beta risk). I do have the following info: AQL, lot size. But I need to determine the sample size and frequency. I tried ANSI level II normal inspection, but it requires a large sample size compared to the lot size (specially for defects classified as critical). Do you have any recommendations? Anything will help!!!
 
S

Sturmkind

#6
I agree with Bev D, the Heretical Statistician. Too many quality professionals have become Hysterical Statisticians when the sample size and frequency was arbitrarily imposed. Grant & Leavenworth's Statistical Process Controls has some of the most lucid explanations for sampling and frequency as well as an excellent chapter on all of the different standard deviation terms used. Well worth the read.

One suggestion from the Red 'X' Shainin disciplines is to decide what you want to 'catch' and control: within part, between part, or lot-to-lot variation?
 
K

kabus - 2011

#7
Actually, the intention is to develop an acceptance sampling used to aid in product disposition. The catch shall be within the lot compared to the establsihed AQL / defect. The thing is that I have read articles related to sampling but, the majority, have been applied to the medical devices or bulk product. I am trying to develop a sampling plan that fits a packaging line (bottles / blisters). I was just reading an article which suggested using the OC curve to determine n (sample size) and ANSI for the acceptance criteria. Can this be done? combine both?
 
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