The answer was found in theory. The Japanese experimented relentlessly to learn the cause of manufacturing defects. If they could only identify the root cause of each and every problem, they believed, then they could design a process to prevent that error from recurring. In this way, manufacturing errors were rarely repeated, quality improved continuously, and costs declined precipitously. In short, what the Japanese proved is that in spite of inherent complexity, it is possible to reliably and efficiently produce quality cars, when you focus on improving the manufacturing process.
