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Re: AQL vs Percentage

From: Stan Hilliard
Date: 09 May 2003
Time: 16:18:19

Comments

Greetings Vandu,

In your example, the plan did what it is designed to do. It accepted a lot that contained 25 defective in 1000. If Pa=95% for a lot that is 2.5% defective, then if you carried out the sampling plan 100 times, you can expect to accept 95 times and reject 5 times.

What if a lot is accepted based on a sample that contains defectives? that is -- more than zero defectives but less than Re defectives in the sample? Generally, the defective items found should not be returned to the lot. The sampling plan accepts or rejects the lot, but not individual defectives. Any defective that is specifically identified should not go to the customer.

What to do about defective items in accepted lots? Individual defectives might be found by the customer in accepted lots. My opinion is that the customer be offered credit for the individual defectives or else replace the defectives items. This is a matter of business relationship and should be agreed upon with the customer at the time of agreeing to the sampling plan. It is best to discuss this before it happens, because with sampling it is almost certain to happen -- unless process capability is perfect.

Is there reason for the customer to complain? If the customer selected the 2.5%, he might have a misconception -- as many do -- as to what AQL means: www.samplingplans.com/aqlprimer.htm

Or, if the process capability is 2.5% defective, the customer might be accepting that for the time-being because he needs your product. The customer might also be preferring that you improve the process.

If there is need to lower the process percent defective, this would involve understanding why/how the defectives occur and changing the process so that defectives are not manufactured. If the cause of the defectives is complex or a solution not obvious, perhaps the "hard" tools like DOE or the "soft" tools like those of Goal-QPC would be appropriate.

Sincerely, Stan Hilliard


Last changed: November 20, 2007