Sampling Plan to determine sample size/frequency based on a Given Rate of Production

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D
#22
Statistically Valid Sampling Plans

QS Auditor wants to see proof that 3 piece smaples for run charts is a statistically valid number. I have researched and pretty much proved he's right - its not. Employees feel five piece checks are too cumbersome. What to do? Thanks for any help-Dawn
 
B

Batman

#23
Hi Dawn!
A subgroup of three may be right for you, five may be better, one may be correct also. Look at your process and determine the correct sample size and frequency. When we start a process, we take many samples in a short time, identify the assignable causes, remove or reduce them, start the process again, take less frequent samples, assess control, and so on, until the assignable causes are eliminated (or at least identified) and we know how long we can run until variation is expected to show up, thus samples are taken prior to this, for possible adjustment.

You can do this for present processes also, if needed.

I find that frequency has more to do with "control" than the sample size, except with multi-cavity tools.
 
D

Don Winton

#24
Dawn,

Batman pretty much sums it up. Do not think of sample size in terms of the MIL-STD-105 mentality. Size and frequency are the keys. Show the assessor your justification in statistical terms where n=3 is valid.

Regards,
Don


------------------
Just the ramblings of an Old Wizard Warrior.
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#25
From: ISO Standards Discussion
Date: Mon, 19 Jun 2000 11:26:19 -0500
Subject: Re: MIL-STD-105D /Khan/Westall

From: "Greg Westall"

FYI
MIL-STD-105D was revised to MIL-STD-105E May 1989 and was canceled Feb.
1995. Suggested replacement is ANSI/ASQC Z1.4

As far as a 1 page plan, I have never seen one that covers all AQL's and
Inspection Levels.

Greg Westall

<< From: "Jawad Khan"
I am looking for a one page matrix with sampling plan per Mil-STD-105D.
Can anyone send me this page? >>
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#26
From: ISO Standards Discussion
Date: Tue, 20 Jun 2000 09:31:33 -0500
Subject: Re: MIL-STD-105D /Khan/Westall/McElhiney
From: Larry McElhiney

>> From: "Jawad Khan"
>> I am looking for a one page matrix with sampling plan per Mil-STD-105D.
>> Can anyone send me this page?


I agree with what Greg says, however, many customers still have things written into their specifications like:

"All inspections will be accomplished using a Mil-STD-105D Level II 1.0 AQL (single normal)"

It is difficult to change their conceptions when we are really only talking about a statistically valid sample plan.

When we were using a C=0 plan, it was referenced to a similar plan as above, but our customers had no trouble accepting it even though it was to the "D" revision because it was, in fact, statistically valid.

Many companies still use a single page sample plan wherein they have a fixed lot size which draws them to a fixed sample size for a given AQL--this is extracted from the appropriate standards and issued by the Quality Engineer to the appropriate work areas. Since the QE can always show how the single page plan relates back to the standards, I have rarely seen it as a problem.

I think that this is the sort of support that Mr. Khan is requesting.

Just my opinion.

Larry
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#27
From: ISO Standards Discussion
Date: Thu, 22 Jun 2000 11:32:23 -0500
Subject: Re: MIL-STD-105D /Khan/Westall/Melvin

From: "Pat Melvin" colorgraphic.net

Even though MIL-STD-105E was canceled and replaced, it may be located at www.variation.com/techlib/standard.html

This site also references the suggested replacement, associated ANSI standards, and ISO standards for Sampling Procedures

Pat Melvin
 
C

Charles

#28
Hi folks,

First time around so sorry if I make a mistake in posting the message. We are now re-visiting all our sampling procedures. I have some comments and queries:

1 - Number of samples - We currently use Mil-Std and we draw from it the number of samples in accordance with the batch size and special/general levels of inspection available. So the number of samples doesn't take into account the OC.

2 - Acceptance Criteria - We, as a supplier, are moving from the use of AQL's to LQ (Limiting Quality). This is because the latter considers the specific level of defects supplied within a specific batch, whereas the AQL considers the average level of defects accepted over a series of batches. The LQ's are obtained after the evaluation of our Finished Product Specification based on the criticality of each defect. From there we work backwards to find the appropriate AQL we should agree with our suppliers in order to have them aligned with our LQ's.

3 - SPC - So for defects generated by our suppliers we will have a sampling plan at the receiving of the goods. Now, for defects produced during our internal processing, we are not totally clear about the roles of acceptance sampling vesus SPC. We thought about implementing SPC, but worry about people requesting a final acceptance sampling of the finished product too. How can we ensure that the number of samples taken for SPC will satisfy an acceptance sampling plan?

Regards,

Charles.
 
T

thebigbear

#29
Sampling Question - Long Story

Hi everyone,
I need some help clarifying a few things and unfortunately this is a long story but it's necessary to help with answering.

I'm currently looking into developing a variables sampling plan for a safety critical item were there is no room for accepting non-conforming items/lots of product. Failure to protect the consumer from poor quality items/lots could mean a death situation. The product in question has a lower specification limit, is destructively tested and the aim is to obviously exceed the LSL to a certain degree to provide a safety margin whilst giving a high process capability. I'm contemplating using a single sampling plan for the process average where the process standard deviation is unknown. Reason: Destructive testing is required and therefore an expensive exercise, also samples only provide estimates of the population mean and standard deviation. Another thing, keeping costs to a minimum is of importance whilst still providing protection to the consumer.

Questions:
1) Is this the right approach? If not could someone please advise of an altenative!
2) Would a chain sampling plan be better to this situation where acceptance c=0.
3) One thing I don't understand with these sampling plans is that they specify that if n samples are taken and no non-conforities are found, accept the "LOT". What is considered a lot? If I have a batch of 100 items and I test 1 in that batch, is the 100 a "LOT" size?

Thankyou for taking your valuable time to read and answer this INCREDIBLY long question.

Nick
 
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