Miner said:

It would depend on your particular controls. If you are using a sampling scheme at each operation, I would not reduce the detection rating at all. If you are 100% inspecting at each operation such that you end up doing a 300 - 400% inspection then you could justify reducing the rating slightly, but not to the level of 100% automated inspection or mistake-proofing. Recall that 100% inspection is about 85% effective. 200% will imrove it slightly, lets say 89%, 300% to 92%. The exact percentages are not important. The point is no matter how many time you 100% inspect, it is not 100% effective.

I'm not sure I understand the reasoning here. First, there is no empirical basis that I know of for the 85%-effective bromide. I think the intent of it has always been to serve as a warning that 100% inspection is likely to be less than 100% effective, and that rational sampling might yield better results more econmically in the long run.

For the sake of discussion, though, let's assume that it's accurate (leaving aside the wealth of variables such as lot size, inspector acumen and the nature of the defect). If we do 100% inspection

*x* times (where

*x *is >1), and the 85% thing is statistically reliable and all else is equal, then the effectiveness is going to be 85% each time. But wait a second...if we have a finite lot size and some known quantity

*y* defectives in the lot, and the first 100% inspection results in

*some* of the defectives being discovered, the second and subsequent inspections should be

*less* effective because there are fewer defects to detect. This assumes, of course, that the purpose of the inspection is to end up with a defective-free lot.

The point here is that we should always try to determine the method of inspection that's most likely to yield the desired results. If the lot is 20 pieces, the defect is glaring and the inspector is well-trained and experienced, 100% inspection might be just the ticket. On the other hand, with a comparatively large lot size, defects that are difficult to detect or evaluate, and inspectors who had seven beers for lunch, sampling is probably a better idea.