From: Stan Hilliard
Date: 4/29/2004
Time: 1:36:54 PM
If you want to accept product at the current process capability but to reject if the process deteriorates in the future, then you need to measure current process capability (Pbar). Then set the AQL of a sampling plan no lower (worse) than Pbar.
On the other hand, if you want to externally impose an RQL or AQL+RQL regardless of the process, you can do that too. Then the process might or might not be able to meet the sampling requirement.
With a new machine or process, one method to make a judgement as to whether it is good or not is to collect any amount of data and then calculate the confidence limits. If the limits are too wide to be useful, increase the size of the sample. This approach can apply to attribute data or variables data. For attribute data, software program TP105 will calculate the estimate (Pbar) and the confidence limits -- one-sided or two-sided limits. See:
www.samplingplans.com/programtp105.htm