to Mikishots: Nice twin otter avatar. I worked for the company building the current Series 400 Twin Otter.
I think you got that correct miki - We were struggling with that very question ourselves. Basically you should be using a sampling method with statistical validation behind it. The mil standards are really good for this kind of thing. You need to search around a bit, there are some very good ones available, both mil std and commerical std. Our problem revolved around the small amount of material we bring in, but if you keep looking you can even find standards that cover small lots. That is what we did, we will find out in January 2013 if we as the knight would say in Indian Jones "You chose wisely". One thing about some of the old military standards is that a lot of the mil specs are no longer supported. So chose wisely.