From: Stan Hilliard
Date: 02 Jul 2003
Time: 21:06:54
Larry said: "We have sampled 1% of our finished product and came up with a 4% defective ... outgoing quality level we've set of 1/1000 (0.1%)"
Your process average is 4% defective and you want the outgoing to be no more than 0.1% defective.
As a general principle, it is not advisable to set the AQL of a sampling plan any lower than the process average -- unless it is a rectification plan, in which rejected lots are 100% inspected to remove all defectives. Before a sampling plan can accept a lot that is 0.1% defective the lot must be 0.1% defective.
The problem is clearly one of the process not being sufficiently capable. I would study the process to determine how to improve it -- it could be out of control. If so, a p-chart could help to trace down the cause of the defectives so that the process can be corrected. Then a sampling plan can be used to hold the gains.
A secondary issue is that once you get the process problem identified and corrected, you will need to consider how to demonstrate low percent defective with small lot sizes.