From: Stan Hilliard
Date: 09 Sep 2003
Time: 12:40:30
Greetings Kirsten, here are my thoughts about the inspection levels, including S1, S2, S3, and S4. of Military Standard sampling plans.
EXPLANATION - HOW: I do not know of any rules to select S1-S4 except to determine what n and Ac that you want, and then to find in Mil-Std-105 the inspection level that gives you a (n,Ac) closest to what you want. The way to determine (n,Ac) is with the oc curve.
www.samplingplans.com/usingoccurves.htm
www.samplingplans.com/aqlprimer.htm
EXPLANATION - WHAT: The initial intent of Mil-Std-105 was for government procurement of war materials in WW2. The customer/buyer (government) would determine the appropriate plan and then to provide Mil-Std-105 to the manufacturer to execute in a "cook-book" fashion. Cook books tell the cook what to do, not why to do it.
EXPLANATION - WHY: The designers of Mil-Std-105 organized the book around the producer's point, not the consumer's point. I think that the reason for this is that in time of war, the first objective is to get the ammunition out to win the war. The issue of defectives is secondary, because in wartime, the military is an expendable resource.
EXPLANATION - CULTURAL: I think that one of the reasons people like Mil-Standard plans for commercial applications is that the Mil-Standard justifies the choice of plan. Many would prefer to avoid having to justify a plan as personal judgement or opinion based on considerations of risk to the consumer and producer. That is why they are given the default choice of using inspection level II.