Not one reference to W. Edwards Deming this entire thread!? You really, really need to investigate his work.
"Deming's teachings and philosophy are best illustrated by examining the results they produced after they were adopted by Japanese industry, as the following example shows: Ford Motor Company was simultaneously manufacturing a car model with transmissions made in Japan and the United States. Soon after the car model was on the market, Ford customers were requesting the model with Japanese transmission over the US-made transmission, and they were willing to wait for the Japanese model. As both transmissions were made to the same specifications, Ford engineers could not understand the customer preference for the model with Japanese transmission. Finally, Ford engineers decided to take apart the two different transmissions. The American-made car parts were all within specified tolerance levels. On the other hand, the Japanese car parts were virtually identical to each other, and much closer to the nominal values for the parts - e.g., if a part was supposed to be one foot long, plus or minus 1/8 of an inch - then the Japanese parts were all within 1/16 of an inch. This made the Japanese cars run more smoothly and customers experienced fewer problems. Engineers at Ford could not understand how this was done until they met Deming." -wikipedia
Read his books! You are asking the wrong questions!