(Apologies for misbehaving by jumping straight into a forum without introducing myself properly
Just to add something to the power of 100% overchecks/inspection
I have just carried out some Human Factors training with our shop-floor, and showed them the following simple bit of 6-sig analysis:
After a bit of discussion with the operators, we agreed it was reasonable to assume 1) that humans were 99% accurate and 2) our typical routing gave a person roughly 50 opportunities to make a human-based error (in actual fact our routings give probably closer to 200, but that is my problem, not theirs!)
Therefore, only 0.99^50 = 60.5% of routings reach dispatch with no errors.
The simplest way of improving this unimpressive figure is through a 100% overcheck. Assuming your inspector is also 99%, then the maths is now 0.9999^50 = 99.5% and through adding an extra op we've suddenly turned the figure into something acceptable.
I have also used this to great effect with top management, who think that Quality Inspection is a waste of resource.