ANSI ASQ Z1.4 - Is the following process acceptable?

F

farasm5

#1
Can I ask a question associated with ANSI ASQ Z1.4?

For instance, AQL=0.65% and code letter J with single sampling plan- normal inspection is used. It requires 80 samples and Ac/Rej=1/2. Here 1 defect is found after inspection. In this scenario, is the following process acceptable?

Step Example
1 In table 2-A initial sampling for AQL 0.65% is 80 with Ac=1/Re=2 (2
defect found)
Go down to 125 with Ac=2/Re=3 (which has AQL 0.65% and code
letter K)

2 Take 45 samples more

3 If 0/45 -> total 2/125 -> Accept

I think that for this purpose, there is double sampling plan and it should be used from the beginning.

My question is ?Is it acceptable or not? If it is acceptable or unacceptable, why? ?
 
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Statistical Steven

Statistician
Staff member
Super Moderator
#2
The short answer is NO. Table II-A is not intended as a double sampling plan. If you sample 80 and found 2 defect, it fails. There is not take additional samples to see if we can accept the lot. If you want to do double sampling plan, go to Table III-A in the same standard. There you will see a sample of 80 acc=0 rej=3 and additional sample for 1 or 2 defects. The additional sample is another 80 units with an acc/rej of 3/4.

Does that help?
 
F

farasm5

#3
Hi, Steven

I appreciate for your response. Then can I ask you one more question?

In this scenario, the lot is rejected after the first inspection.

If the defect can be inspected with a non-destructive method, there is an option that the rejected lot can be re-inspected 100% and remove the defective units.

But if the defect cannot be inspected with a non-destructive method, is there any way, which is statistically valid after rejection, in order to reinspect the rejected lot? Or do we have to scrap the rejected lot?

Thanks.

Peter
 

Statistical Steven

Statistician
Staff member
Super Moderator
#4
Hi, Steven

I appreciate for your response. Then can I ask you one more question?

In this scenario, the lot is rejected after the first inspection.

If the defect can be inspected with a non-destructive method, there is an option that the rejected lot can be re-inspected 100% and remove the defective units.

But if the defect cannot be inspected with a non-destructive method, is there any way, which is statistically valid after rejection, in order to reinspect the rejected lot? Or do we have to scrap the rejected lot?

Thanks.

Peter
Peter

You can always do 100% inspection and cull out the defective units. As a matter of fact, if you had a non-destructive method for inspection, you do not even need to do sampling. Once your sampling plan fails the lot, there are no other sampling besides 100% sampling that would be statistically valid in my opinion.

There are some who would propose a second sampling at a more stringent AQL (say 0.1) instead of the inital 0.65 level, but the standard does not have that option. You can convert to a double sampling plan, but I personally do not that option and perfer it is determine apriori not after a failure is found.
 
F

farasm5

#5
Hi, Steven

Thank you very much for your help. I hope that in the future, I have another chance to chat with you.

Have a nice weekend.Thanks again.

Best Regards,

Peter
 
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