Can ANSI Z1.4 sampling plan be used for sampling alone without the Acc/Rej criteria?

#1
Hello all,

Is it normal, or acceptable to modify the acc/rej # when choosing a sampling plan? For instance, I have a process that currently is a Level II, @ .65. However the ac/re value is 2/3.

Can I just use the ANSI Z1.4 for picking sample size only and modify the ac/re criteria to 0/1 and still be considered as following ANSI Z1.4?

Much appreciated,
John
 
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David-D

Involved In Discussions
#2
Re: Can ANSI Z1.4 sampling plan be used for sampling alone without the Acc/Rej criter

I personally wouldn't say that modifying the accepr/reject criteria would be in keeping with Z1.4. In you're approach, you are changing to 0/1, which is in practicle terms is shifting your AQL to the left on the chart to a smaller value. You are asking for higher quality fron your supplier. There are many practicle reasons to change sampling from standard tables (or create your own), but it isn't appropriate to continue to describe it as 1.4.

David
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
Re: Can ANSI Z1.4 sampling plan be used for sampling alone without the Acc/Rej criter

Hello all,

Is it normal, or acceptable to modify the acc/rej # when choosing a sampling plan? For instance, I have a process that currently is a Level II, @ .65. However the ac/re value is 2/3.

Can I just use the ANSI Z1.4 for picking sample size only and modify the ac/re criteria to 0/1 and still be considered as following ANSI Z1.4?
the simple answer is no.
the detailed answer is that the sample size AND the Accept/Reject number are what dictate the quality level (AQL and/or RQL) and confidence of the plan. if you change the Acc/Rej numbers you are changing the quality level and cconfidence level so NO you are not following or using the standard.

What do you really want to accomplish? we can help you with that...
 

Statistical Steven

Statistician
Staff member
Super Moderator
#4
Re: Can ANSI Z1.4 sampling plan be used for sampling alone without the Acc/Rej criter

Hello all,

Is it normal, or acceptable to modify the acc/rej # when choosing a sampling plan? For instance, I have a process that currently is a Level II, @ .65. However the ac/re value is 2/3.

Can I just use the ANSI Z1.4 for picking sample size only and modify the ac/re criteria to 0/1 and still be considered as following ANSI Z1.4?

Much appreciated,
John
John

Why MUST you consider yourself following ANSI Z1.4? If you want a c=0 plan, you will be able to reduce the sample size from Z1.4 in most situations to maintain the same AQL. Look at the OC curve, see how the AQL and LTPD are impacted by reducing your acc/rej numbers.

Just so you are clear, if you say you are following Z1.4, that implies you are using the switching rules.
 

Mike S.

Happy to be Alive
Trusted Information Resource
#5
Re: Can ANSI Z1.4 sampling plan be used for sampling alone without the Acc/Rej criter

Many aerospace customers, the US GOV included on occasion, will instruct a supplier to use Z1.4 sample rates but make the Ac/Re number 0/1. You are not technically following Z1.4 rules then (you are being tougher).

But if you call it a 0.15 AQL and follow the other Z1.4 rules (switching, etc.) you can say it is following Z1.4 as the sample rate is the same with Ac/Re 0/1.

An alternative is to use a true accept on zero sampling plan like Squeglia (ISBN 978-0-87389-739-6)
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#6
Re: Can ANSI Z1.4 sampling plan be used for sampling alone without the Acc/Rej criter

Interesting - there is a rather similar discussion going on about arbitrarily reducing the the acceptance numbers to zero across the board . . .
 
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