Article Revised: March 26, 2019
If you’ve ever read The Toyota Way, or even Taiichi Ohno’s book Toyota Production System you’ll be immediately struck by one observation: quality is simple. Toyota resolves most quality issues quickly because they limit the big quality killer: inventory. Inventory kills quality by placing a time-delay between the appearance of a quality problem and it’s discovery. When the customer–who may be the end user or the person at the next step in the process–finds a quality problem at Toyota, it has just been produced. This makes it far easier to diagnose the root cause and take action to eliminate it. When inventory exists there is a delay of hours to months between the creation of the problem and the discovery of it. By then the cause of the problem may be lost. Not to mention that there may be many items with the problem in inventory.
It’s not enough to simply put a control chart on the process in real time. Sure, control charts can help identify quality problems when we know what to look for. But they’re no substitute for the actual customer using the product. After all, quality is defined by the customer and they’re in the best position to identify what is and what isn’t a problem.
Complexity arises when we try to divine the root cause of a problem where there is a time lag. We bring heavy duty statistical methods, data mining, root cause analysis, etc. to bear because the trail has gone cold. While these methods may work at times, it would be far simpler if we could simply look at the process as it creates the problem. KISS, keep it simple stupid, is the way to do this. And one of the best ways to keep problem solving simple is to minimize inventory.