AQL vs. Cpk - Sample Size and Reduced Sample Size (2024)

Re: AQL vs Cpk

Mike Hammill said:

DISCUSSION
AQL (Acceptable Quality Level) assumes no knowledge of the capability of the process that produced the part being sampled. What if the underlying performance capability is known? What if the capability is Cpk = 2.0 (or 6 sigma or 3.4 defects per million opportunities). Does it still make sense to continue to test with a specified AQL Level? Sample size and the decision to Reject or Fail to Reject the lot is based upon Percent Defective (or Binomial Distribution) with the null hypothesis (Ho) being, % defective is not greater than 1% (for an 1.0 AQL) and the alternate hypothesis (Ha) being greater than 1% defective. A 1.0 AQL is 10,000 DPMO or 3.83 sigma.

Or asked another way, if our customers wished a 5.5 sigma performance, or 100 DPMO, then this would be a 0.01AQL. What would the sample size be for that? 100%?

QUESTION 1
Is the sample size the same between parts/features with both known and unknown capability? Or stated differently, what is the right sample size for parts with known performance capability (Cpk and/or Ppk)?

QUESTION 2
Is there a specification/method that defines a reduced sample size for processes with known capability. The purpose of the sample would be to ensure that the process is stable (SPC might be a method to ensure this).

Click to expand...

Mike - Welcome to the cove. You are in the right place to get a logical answer to your questions. You may want to consider the use of skip lot inspection / sampling method once your process has been proven to be statistically capable. We employed this technique in our receiving inspection of incoming components and it worked very well for us.

Re: AQL vs Cpk

O.k.

So what drives the decision to skip a lot (or many lots and how many)?

Guess I'm not looking for a black and white answer, but a general line of logic that would pass a litmus test for those "locked-in" to a current line of thinking. I can do the sales pitch, just need some help putting the pitch (argument) together.

Mike

Re: AQL vs Cpk

When you are dealing with very small PPM levels, sampling of any kind is ineffective.

You have two basic choices. Either 100% inspect to attempt to detect the small number of defects, or trust in your Cpk = 2 capability, control your process with SPC to ensure that it does not drift, and do not inspect at all.

Re: AQL vs Cpk

Miner,

You make a good point.

What should be the sample size for the SPC? 1? 2? Depends on the Cpk? First, middle, last piece?

Ultimately I'm trying to reduce the Appraisal COQ without increasing risk of failure.

Thanks,

Mike

Re: AQL vs Cpk

The subgroup size and frequency should be based on the process. Read up on Rational Subgroups to help determine this. I cannot really advise you without knowing a lot more about your process.

Re: AQL vs Cpk

Thank you. That is helpful.

Re: AQL vs. Cpk - Sample Size and Reduced Sample Sixe

Dear all,

I would like to continue the post because I have a similar problem that maybe link to that.

What I'm used to do is:
- validate a process (check process capabilities)
- choose an AQL in regards of the process capability

I was used to used the following table:

This table is of course not from me, but from a medical company and is stated as a guideline.

But my question is: HOW THE LINKS IS MADE BETWEEN CpK and AQL ???

Is there any calculation, curves, tables or whatever that is here to explain a little bit this guidlines?

Thanks for any helps

Re: AQL vs. Cpk - Sample Size and Reduced Sample Sixe

Up!

It is understood that with a 6 sigma capabiltiy sampling is inefective, but question 1 is still open.

Does somebody is able to explain what method they use to select an AQL against a CpK ? Does someone is using the same exemple of table shown above?

Does somebody knows any rational behind this table?

Thanks

Re: AQL vs. Cpk - Sample Size and Reduced Sample Sixe

Palt88 said:

Cpk < 1.0 100%

Cpk >= 1.0 AQL 1.0

Cpk >= 1.33 AQL 2.5

Cpk >= 1.66 AQL 4.0

Cpk >= 2.0 No sampling​

​

Click to expand...

This table does not make sense to me!
AQL (Acceptable Quality Limit) is essentially the % of defective material which could be found in a lot before the lot is rejected. This table is saying that as your capability improves and therefore the % defective within the population decreases then it is OK to accept a lot with more defects.

The Cpk and AQL are contradicting each other.