Double Sampling Plan Explanation for *

T

ThaRide

#1
We have a product line with a lot size of 500. The inspection calls out for Level S-3, AQL=1.5% (Letter D). For a single sampling plan, that is 8 samples. Accept on 0 rejects, Reject on 1. I understand that much.

When a failure occurs, it has been suggested to then use a double sampling plan to further sample the lot to determine if it can be released. When I look on the other side of the AQL Inspectors rule, for 1.5% (Letter D), an asterisk is there and the notes suggests to use the single sampling plan on the other side. Since we already inspected 8 samples and had 1 reject, do we just pull another 8 samples (doubling the initial sample size) and release the lot if there are no more rejects?

The Double sample plans seem easy if you have a letter where the Acc-Rej is already defined 0-2, 0-3, but there doesnt seem to be a very good explanation when the * is identified.

Any help woudl be appreciated.
 
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T

ThaRide

#3
Thanks. It says for Double Sampling to use 20 samples. Which means that I ignore the reference to the asterisk. What is the purpose then of the asterisk? If I am using the Double Sampling plan, why does it even bother referring me to the Single Sampling plan?

So, lets say I did initially use a Single Sampling plan (8 samples, Acc-0, Rej-1) and had 1 failure. We decide to re-evaluate the lot using the Double Sampling Plan. I should then pull another 12 samples to make sure there is not another reject and then pull an additional 20 to complete the Double Sampling? Is that correct?
 
A

arios

#4
Thanks. It says for Double Sampling to use 20 samples. Which means that I ignore the reference to the asterisk. What is the purpose then of the asterisk? If I am using the Double Sampling plan, why does it even bother referring me to the Single Sampling plan?

So, lets say I did initially use a Single Sampling plan (8 samples, Acc-0, Rej-1) and had 1 failure. We decide to re-evaluate the lot using the Double Sampling Plan. I should then pull another 12 samples to make sure there is not another reject and then pull an additional 20 to complete the Double Sampling? Is that correct?
If a double sampling plan is being used which initially asks you to sample 8, Acc-0, Rej-1 and you find one defect then you reject the batch, and there is no need to keep sampling. A disposition will then need to be made with the lot that was rejected: scrap, rework, sort, etc.

On the other hand if a double sampling plan provides you for example with a criteria like: Acc-0, Rej-2 and you find 1 defect on the initial sample you can't yet make a decision over the batch since it is between the Acc-Rej criteria, and that is when you use the second sampling. On the second sampling you will count the accumulated defects detected from the first and the second sample and compare the result vs. the Acc-Rej criteria for the second sampling, and that is when you make a decision. If no more defects are found you can accept the batch and take out the one piece defective

When you use a double sampling plan and you are required to examine 8 units on the 1st sampling, and 20 on the 2nd sampling, then you sample 8 and then 20, not 12. Sample size is not cummulative, but defects are.
 
T

ThaRide

#5
Thanks for all of your help, it is greatly appreciated. So, for a lot size of 500, we would need to have selected Level I, Level II or Level III to have a chance at accepting the lot because all three of those Inspection levels would have pointed me to 20 samples Acc-0, Rej-2. If we use Level S-1, S-2, S-3 or S-4 that points us to the asterisk, which in turn points us to the Normal Sample Size of 8 samples Acc-1, Rej-0.

Since there is a chance of failure, what we would like to do is to be able to react when we do get a failure from Acc-0, Rej-1 and still state our confidence in the lot. If I am looking at AQL 1.5%, will I have the same confidence in the quality of the lot if I pull 8 samples (Acc-0, Rej-1) or if I pull 32 samples (Acc-1, Rej-2)?
 
A

arios

#6
Here is my "5 cents":

Dr. Kaoru Ishikawa on his book titled "Guide to Quality Control" (Chapter 12) provides guidance on the selection of the inspection levels.

I will here quote him:

"Inspection level I: when the inspection cost is comparatively high
Inspection level II: ordinary cases
Inspection level III: when inspection costs are low
Inspection level S-1 to S-4: when the cost of destructive testing is high"

I know that there are folks in Elsmar with very good background in statistics and sampling techniques who can provide us with further guidance but at least I recommend the use of bibliographic support to define or select the inspection levels and justify the use of one or another approach.
 
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