The word 'quality' is generally used in connection with expensive items and luxury goods. Nowadays, the quality of their product is what suppliers and sub-suppliers live by. Customers, world over, demand and expect it.
In manufacturing, much effort has gone into 'engineering in' quality by carefully optimising the design and production methods. However, a variety of manufacture and assembly faults often result in unacceptable noise and vibration levels in manufactured products. There is still only one method that makes absolutely sure that every item coming off the production line fulfils expectations. That method is production line testing, where every unit produced gets checked as part of its production cycle.