Attribute Sampling Plan help wanted

D

dipeshjayswal

#1
Greetings All-

We have following situation for acceptance sampling and we use standard for sample size.

An order of 1000 units came in one container containing 10 sub-containers, 100 each. Now required sample size is "n" per the standard, how should we select the sample and what is it call?

Currently we select equal (similar) # of units from EACH sub-container, but our auditor says it is not random. What term should we use if we want to continue with the same practice.

Thanks in advance!
DJ
 
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Jim Wynne

Staff member
Admin
#2
Re: Attribute Sampling Plan

You should understand that if you're using a standard sampling plan, it's based on the assumption that the sampling will be random, meaning that each member of the 1000-piece population has an equal chance of being chosen.

That's not always practical, so if you need to call your sample something other than "random," just call it a sample.
 
D

dipeshjayswal

#3
Re: Attribute Sampling Plan

Thanks Jim,

What is the acceptable method if we want to have random sample?

Can we use stage wise random sampling?.
That is, selecting sub-container (s) at random (first stage random sample). And then selecting sample from that sub-container.

Is there any standard plan which allows stage wise random sampling?

Please help.
DJ
 

Tim Folkerts

Super Moderator
#4
There is a term known as "stratified sampling" -- http://en.wikipedia.org/wiki/Stratified_sampling

Basically, you divide the population into groups (strata) and then sample from each group in proportion to its relative size. In this case, each stratum would be a sub-container.

So you could tell your auditor you are doing a "random sample stratified by sub-containers." That should impress (or confuse!) the auditor enough to be acceptable.

With your example, you have 1000 parts. Suppose that AQL = 2.5 and you are using Normal, Level 2 inspection. The table says to sample 80, or 8 from each sub-container for a stratified sample. You would reject with 6 bad parts total.



I don't know of any standard plan that lets you sample from only some containers. One though I had before is to first follow a standard plan for sampling sub-containers.

With your example, you have 10 sub-containers with 100 parts each. Suppose again that AQL = 2.5 and you are using Normal, Level 2 inspection.

  • For a sample of 10 sub-containers, the tables say to inspect 5 sub-containers.
  • For each sub-container of 100, the tables say to sample 20 parts. 2 bad parts in any sub-container would reject the sub-container. And one rejected sub-container is enough to reject the whole lot.
 
U

Umang Vidyarthi

#5
Re: Attribute Sampling Plan

Thanks Jim,

What is the acceptable method if we want to have random sample?

Can we use stage wise random sampling?.
That is, selecting sub-container (s) at random (first stage random sample). And then selecting sample from that sub-container.

Is there any standard plan which allows stage wise random sampling?

Please help.
DJ
Yes you can. This is known as 'Stratified Sampling'. Here you take the sample from each sub-population. You have to judiciously plan the 'strata' or 'sub-group' within the population.

Umang :D

oops! Tim has beaten me on this.
 
D

dipeshjayswal

#6
Thanks Tim and Umang,

I thought of using the term "Stratified Random Sampling", but we do not have any considerable or distinguishable property which can make each sub-container as strata. If I am not wrong, in stratified sampling we anticipate at least one property which changes from strata to strata. That means, homogeneity within strata and heterogeneity between strata.

Here, in my example variability between and within sub-containers are supposed to be the same.

I am thinking of writing following statement in my sampling procedure:

"If a final container is a collection of sub-container of products then a random sample of most appropriate size (depending upon the total n required and # of sub-containers) from each sub-container will be selected to have required total sample size n per standard sampling plan"

Please suggest better wording if you feel above statement is not clear or misleading.

You people are awesome!

Thank you again!
DJ
 
U

Umang Vidyarthi

#7
Thanks Tim and Umang,

I thought of using the term "Stratified Random Sampling", but we do not have any considerable or distinguishable property which can make each sub-container as strata. If I am not wrong, in stratified sampling we anticipate at least one property which changes from strata to strata. That means, homogeneity within strata and heterogeneity between strata.

Here, in my example variability between and within sub-containers are supposed to be the same.

I am thinking of writing following statement in my sampling procedure:

"If a final container is a collection of sub-container of products then a random sample of most appropriate size (depending upon the total n required and # of sub-containers) from each sub-container will be selected to have required total sample size n per standard sampling plan"

Please suggest better wording if you feel above statement is not clear or misleading.

You people are awesome!

Thank you again!
DJ
You got the bull by the horns! Go ahead and best of luck.

Umang :D
 
H

Hodgepodge

#8
Currently we select equal (similar) # of units from EACH sub-container, but our auditor says it is not random. What term should we use if we want to continue with the same practice.
How does the auditor define "random"? If there is no definition of "random" provided by your customer, then your company gets to define the process for the purposes of product verification within your company. Without knowing all the details, it is likely that the top sub-container holds a random sampling of the lot. Considering the random possibilities of handling during manufacturing, cleaning, inspection, & packaging, each container is not likely to represent the product in order of manufacturing (not counting assembly line production, of course). I would not adopt this approach, but theoretically, it is possible.

Instead of making up some random definition of "random", a quick look at the government's procurement methods can help you set up something quickly.

For instance, MIL-STD-1916, DOD Test Method Standard, DOD Preferred Methods For Acceptance of Product, Section 4.2.4 Sampling of lots or batches (my emphasis in bold):

"4.2.4.1 Selection of units.
Units of product drawn from a lot for a sample shall be selected at random from the lot without regard to their quality. Random sampling requires that each unit in the lot, batch, or production interval has the same probability of being selected for the sample.

4.2.4.2 Representative (stratified) sampling.
When appropriate, the number of units in the sample shall be selected in proportion to the size of sublots or subbatches, or parts of the lot or batch, identified by some rational criterion. When representative sampling is used, the units from each sublot, subbatch, or part shall be selected at random.
4.2.4.3 Process of sampling.
A sample may be drawn after all units comprising the lot or batch have been assembled, or sample units may be drawn during assembly of the lot or batch, in which case the size of the lot or batch shall be determined befored samples are drawn. When the lot or batch passes the sampling plan, such lots or batches are acceptable and may be submitted to the Government."

This is such a ticky-tack finding! What would seem more important to me is how you determine the appropriate sampling size.


MIL-Standards are free @ http://assist.daps.dla.mil/quicksearch/.​
 
Last edited by a moderator:
D

dipeshjayswal

#9
Thanks a lot Hedgepodge,

The way we do sampling (Please see my initial post for reference) does not fit in any of these definition of Random Sampling and Stratified Sampling.

When we select units from particular sub-container, the probability of selection is zero for all other units which are not in that sub-container. Therefore, it is a violation of the assumption of random sampling.

Here, we do not have any rational criteria to differentiate sub-containers. Therefore, we can not use the word "stratified or representive sampling" for this technique.

I though of using word "Pseudo Random Sampling", but quality people argued that this can not be used in "Quality" as it sounds like "fake random sampling". As I know in statistics, people use word "Pseudo" for kind-of situation like "Pseudo Randomization" which is not pure randomization but generated trough computer using seed number.

Finally, i end up with writing following statement for the sampling we do:

"If a final container is a collection of sub-container of products then a random sample of most appropriate size (depending upon the total n required and # of sub-containers) from each sub-container will be selected to have required total sample size n per standard sampling plan"

Would you guys please reword if above statement is misleading or not clear?

Thank you very much in advance
DJ
 
#10
I agree with Jim. I believe your auditor doesn't really understand. Just say you sample. On the premise that you are buying parts which are supposed to be almost identical, then you can pull examples from just about anywhere.

The fact is that sampling plans are not really suitable for receiving parts from a supplier unless you - and the supplier - have done all the requisite studies. Therefore - and I'm using my experience of doing similar over 4 years of running Receiving Inspection, just pull your sample from wherever you want and don't call it 'random'. Also, ask for an auditor who doesn't make such comments!
 
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