Setting AQL (Acceptable Quality Level) on C=Zero inspections

[Norman V]

Here's a general question:

We use a C=Zero sampling system for our incoming inspection, using "associated AQLs". What is the best way to determine which AQL to select for a particular part being inspected under this type of system? I have a Quality Engineer that wants to develop a set of "standard AQL levels" for each type of product being inspected.

For example: Rubber seals would always have an AQL of 1.0, nuts and bolts, 10.0, and so on.

Does anybody within the sound of my voice have any input into how this should be handled? We're trying hard not to be arbitrary.

Thanks.

Norman V.

 

[Bill Ryan - 2007]

Sorry, I've got a major disconnect going right now but I'm responding anyway (they say the first thing that goes.......).

I don't see how an AQL has anything to do with a C=0 "sampling plan". By definition, doesn't AQL imply defective/nonconforming parts are OK?

Bill

 

[Norman V]

Ok, let's clarify

In the traditional sense of AQL, (MIL-105-E), you are correct, AQL determines (among other things) how many rejects you can accept in a lot. In a C=Zero system, the "associated AQL" merely is used in a table to help you determine how many samples to pull based on lot size.

The AQL levels are "associated", because the Operating Characterisitic (OC) Curves would look about the same in terms of a percent defective for each AQL level, even though the acceptance number is Zero.

My main questions was, what is the best way to decide how to set the AQL level during the Quality Planning process, aside from just picking an AQL arbitrarily?


Thanks,

Norman V.

 

[Mike S.]

Since it is incoming inspection, I'd sit down with your internal customer(s) who will use each part and discuss it with them. Discuss what impact a defect will have on them - i.e. if there is 1% defective how would that affect you and downstream operations, etc. Also, what has been the incoming quality level in the past, and was it "acceptable" to them in the past. Also consider time of inspection, etc. Some will rant and rave and say no defect is ever "acceptable", but in the real world, IMO, zero defects is a pipe dream. JHO. Hope this helps.

 

[Craig H.]

Hi

Dr Ken Stephens (Applied Acceptance Sampling, available through ASQ) gives the following steps:

1. Determine and list each quality characteristic
2. Determine the product unit.
3. Develop and specify the test method.
4. Determine the criteria for conformity.
5. Determine and list the calssification of nonconforming units or nonconformities and/or groups of nonconformities.
6. Establish an Index Quality Level (AOQL, etc).
7. Determine if sampling/acceptability should be based on lot inspection, on-line (continuous, etc.


I have excerpted here, and he goes on to more specifics. We used this textg for one of my classes at SPSU, and I can say that if you want to find out about acceptance sampling, this is the text.

The class was mind boggling.

Hope this helps.

 

[Norman V]

Mind Boggling?

Craig,

When you say "mind-boggling", do you mean that in a good way or a bad way?


Anyhow, I saw that book in the ASQ catalog, and it does look good. The CD ROM that accompanies it seem interesting as well.

Maybe I'll put it on my Christmas list.


Thanks,

Norman V.

 

[Craig H.]

Norman:

Mind boggling in both a good and bad way, really.

Before this class, I assumed that Dr. Deming ment that inspection was bad. Not so. What he said, and ment, was that DEPENDANCE on inspection for quality is bad. We still inspect, but our process is such that we don't make bad stuff.

In a bad way, we actually designed acceptance sampling plans - those are some of the major files on the CD. Not for those with less-than-complete statistical backgrounds, IMHO. The class was the most difficult in the SPSU Master's program. We used the draft for Dr. Stephen's book, and he taught the class (online). He is retired now, but his list of publications is impressive, and this particular book is highly recommended.

 

[Douglas E. Purdy]

But the question still has not been answered, unless it is in The BOOK? I believe the act of deciding is a step (#6), but are there any generally accepted parameters in making that decision, OR do we have to read the book?

 

[Craig H.]

Douglas:

In short, if you want an optimum answer, read the (a) book, and use it to decide what is best for your situation.

Acceptance sampling is like a lot of other tools - it can be powerful, but only if it is used in a manner that fits the situation. I could give you a "canned" answer, but without knowing the situation, it would likely be worse than the "arbitrary" alternative.

Sorry I can't be of more help, but to give an answer here might do more harm than good.

 

[Kevin Mader]

Craig,

Good information.

Kevin