How to choose the correct General Inspection Level & AQL level...?

[Wesley Lai]

I hope somebody can help me out with this main question:"How to choose the correct General Inspection Level & AQL level and what is the reliability of the outcome?"

We do many inspections by General Inspection Level II + AQL 2.5. Only knowing that this standard is commonly used.

Correct me if I am wrong. AQL 2.5 means that we can accept a defect rate of 2.5%, which means 97.5% of the goods are non defect.

And by choosing General Inspection Level II, means doing a sample inspection and the result compared with the AQL table will determine rejection or acceptance of the goods. What is the reliability of the decision based on these statistics?

Let's say, I do non destructive tests on a batch of 151-280pcs. I choose to accept AQL level 1.0. My test results should be 95% accurate/reliable. Which General Inspection Level must I choose?

Looking forward to have your opinion.

Thanks in advance~!

[Tim Folkerts]

Hi Wesley, and welcome to the Elsmar Cove.

Unfortunately, there is no simple answer to your question. The Z1.4 (formerly known as MIL-STD-105) sampling plans are not based on specific statistical criteria.

The AQL level is a defect rate that will usually get accepted. So if your have AQL = 2.5, then lots with 2.5% defective will probably get accepted. (The probability depends on all sorts of factors. For normal inspection, the number is typically around 95%, it can be from roughly 90-98% as I recall). The batches have to be much worse (typically 3-10 times worse) before you can be moderately sure of rejecting them.

The only real way to judge the sampliong plans is to look directly at the OC curves and tables at teh back of the standard.


"a batch of 151-280pcs...AQL level 1.0 ... 95% accurate/reliable. Which General Inspection Level must I choose?"

For a given AQL, you can look up all the different sampling plans, then find the defect rate that will be accepted 95% of the time and the defect rate that will be rejected 95% of the time.

CODE..SAMPLE...AC..RE...95%......95%
..........SIZE................ACCEPT.REJECT
E.........13..........0....1...0.394....20.6
H.........50..........1....2...0.715....9.14
J..........80..........2...3....1.03.....7.66
K.........125.........3...4....1.09.....6.20
L.........200.........5....6...1.31.....5.26

For your example, the only sample sizes you would get for a batch of 151-280 are 50 (Level II & III) or 13 (all the other Levels)
for normal inspection. That doesn't give you very much flexibility!

Another option is to design you own sampling plan. Select the defect rate you will accept 95% of the time, the defect rate you will reject 95% of hte time, adn find a sampling plan that will work for your needs.


Tim F

[gglntn]

Concerning AQL, we use 1 gallon plastic bottles to package product. Currently bottles are simply brought in the door with no incoming inspection, sent to the production floor, and used, generally within 24 - 48 hours. During production, as the operator manually puts the bottles on the conveyor belt leading to the fill station, (S)He inspects each one for defects, debris, etc. Defective bottles are immediatlely counted and discarded.

Should QA be doing incoming inspection, and if so, how would we go about chosing an accept/reject level for the lot, knowing that further down the line bottles will be 100% inspected by the production operator.

Why would be ned to develop an AQL to reject the lot? All that would do would be to add an additional step. Thnx

[davidzhao]

AQL is related to you and your supplier also.Let's say AQL was set to 1.0,if the vendor's quality level is 5%(reject ratio),you'll not get approval of inwards.
So when you set the AQL,check how much failure you can accept(normally the failure will be graded as A,B,C etc which related to safe/regulator,function/dimension,appearance,etc), and check with vendor where vendor is. If vendor can not meet the AQL you set and you insist on that,need vendor do 100%inspection before deliver.
As long you have a inspect station(visually) on site,you can accept more which kind of failure will be caught up by inspector.
Any way,it's related to your company, and also your vendor,and it's active and will be updated when things improved.

[Renaud]

I agree with Tim's response.
To complement it: about your sentence "by choosing General Inspection Level II, means doing a sample inspection and the result compared with the AQL table will determine rejection or acceptance of the goods":
The general inspection level II is what the norm calls the "normal" level. If you go to a reduced (I) or tightened (III) level, the discriminatory ability of your inspection will be different. You can switch to one of these levels if you have strong hints--one way or another--about the quality level (e.g. if the same supplier's goods were rejected more than twice over the last 5 inspections, you should go to level III.
I hope it helps.

[SteveK]

In the past for plastic moulding etc the AQL level allowed different AQLs for different defect levels i.e. Critical (e.g. 0.65), Major (e.g. 2.5), Minor (e.g. 6.5). This miight give you some extra flexibility, depending on how you classify the defects. Or am I just complicating matters?