From: Stan Hilliard
Date: 03 May 1999
Time: 22:50:42
> What is the influence of Level I,II,III on OC curve
Those levels are from standards Z1.4 and 105. As you change from inspection level I to level II to level III the OC curve gets steeper (more vertical). Steeper OC curves give you the ability to discriminate between levels of percent defective that are closer together. You pay for this more precise discrimination by having to take a larger sample size.
The inspection levels reflect holding the AQL approximately constant and changing the consumers point. As you change from inspection level I to level II to level III the consumer's point(RQL,Beta) moves to the left, closer to the producers point. You can get an idea what was done to generate the levels by looking here(the URL is case sensitive):
http://www.samplingplans.com/modern3.htm#COMPARE
That is my technical answer. My practical opinion is that it is better not to use the inspection levels at all, and instead choose the both consumers and producer's points directly. This will pin down the OC curve exactly where you want it. Software program TP105 makes it easy.
The inspection levels are a "cookbook" way to get a sampling plan without needing to understand what the risks are and what protection the plan offers to the producer and the consumer.
Sincerely, shilliard@samplingplans.com