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Determining Sample Size - AQL & LQ

S

SLEEPYMUM

#1
Hi Folks,

My brain is currently mush from debating sample sizes, AQL/LQ & Pa.:(

Can some one please help in determining what sample size we should be using for the attribute inspection.

Customer gave the following spec:
1.5% AQL with and 8.2% LQ at Pa = 0.10

I do understand AQL/LP & Pa but trying to tie it together is confusing.

Our batch sizes will typically be 105,000

Thanks in advance for your help!
 

Mike S.

An Early 'Cover'
Trusted
#2
Re: Sample Size AQL & LQ

A quick look at ANSI Z1.4 shows you could use single normal sampling, Gen Insp Level II, 1.5% AQL, sample size 500, Ac = 14, Re = 15, LQ = 4.0.

You could use an accept on zero plan with smaller sample, too.

EDIT: Looks like the plan that matches closest is level S4 (letter J) 80 pc sample size, LQ = 8.2, Ac = 3, Re = 4
 
S

SLEEPYMUM

#3
Re: Sample Size AQL & LQ

Thanks for that, but whats confusing me is the customer is requesting 8.2% LQ...

Does this change the sample plan and accept rate?
 

Mike S.

An Early 'Cover'
Trusted
#4
Re: Sample Size AQL & LQ

Thanks for that, but whats confusing me is the customer is requesting 8.2% LQ...

Does this change the sample plan and accept rate?
No, it is part of the characteristics of the plan. Whoever wrote the requirement should have just given you the sampling plan -- looks like they were testing you. Is this an academic pursuit or a real-world one?
 
S

SLEEPYMUM

#5
Re: Sample Size AQL & LQ

No, this is for a new customer who is coming on board.
We currently use C=0 acceptance plans for our other customers, so the fact that AQL & LQ are listed in this new customer spec is what is confusing.
 

Mike S.

An Early 'Cover'
Trusted
#6
Re: Sample Size AQL & LQ

If the customer did not object, you could use a 1.5% AAQL AOZ plan with a 56 piece sample size and LQ of 4.1 but it may be harder to meet than the Z1.4 plan that seems like it is the one the customer had in mind based on the LQ they quoted.
 
S

SLEEPYMUM

#7
Re: Sample Size AQL & LQ

Hi,

If I put the data into Minitab, I am getting a sample size of 80, reject on 4.... :(

This then calculates the OC curve....

DO these reference the ANSI tables?
 

Mike S.

An Early 'Cover'
Trusted
#8
Re: Sample Size AQL & LQ

Hi,

If I put the data into Minitab, I am getting a sample size of 80, reject on 4.... :(

This then calculates the OC curve....

DO these reference the ANSI tables?
Not sure I understand your question. But the Minitab output basically duplicated the Z1.4 plan I mentioned above from what it sounds like.
 

David-D

Involved In Discussions
#9
Re: Sample Size AQL & LQ

Basically they're providing you the two critical points to curve-fit your sampling plan's OC curve to. There should be a broad range of sampling plans capable of doing that. You could also consider variable sampling plans if applicable (as with Z1.9). It doesn't seem to me that they're prescribing a specific plan from Z1.4 (or MIL-STD-105) but are instead describing the levels of customer and supplier risk they're looking to undertake. You could probably find a canned Z1.4/Z1.9 plan thats a pretty good match for your/their needs, or you could create one of your own.

David
 
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