What is Five Whys?

The practice of asking "why?" repeatedly whenever a problem is encountered, in order to go beyond the obvious symptoms and thereby discover the root cause.

For instance, Taiichi Ohno gives this example concerning a machine that stopped working (Ohno, 1988):

1. Why did the machine stop? There was an overload and the fuse blew.

2. Why was there an overload? The bearing was not lubricated as it should have been.

3. Why was it not properly lubricated? Because the lubrication pump was not pumping enough.

4. Why was the pump not pumping enough? The pump shaft was damaged and rattling.

5. Why was the shaft damaged? Because there was no guard and metal chips were accumulating in the pump.

Without the successive questions, managers would simply replace the fuse or the pump and the failure would happen again. The number five in itself is not what matters. What matters is to keep asking until the root cause is identified and eliminated.

Related Article: "Five Whys" - Author: Michael Ballé - Lean Institute Brasil. Read the full article here.

"We can see it shifting from the machine's demand to the lubrication of the bearing, to the pump, to the wear of the shaft, to the protection of the shaft: in order to solve the problem, we no longer control the machine's overload, but the entry of metal scraps into the pump. This classic example also shows why the "five whys" are so complex: in the same situation, anyone with less intimate knowledge of the workings of the machine could have answered differently and steered the "five whys" to a completely different conclusion. The measure of the evaluation of a successful "five whys" analysis is when it leads you to change your mind about which driving parameter to control to ensure process capability.

This, in turn, reflects exactly the kind of deep thinking that the "Thinking Production System" is supposed to develop. Most technical problems have multiple causes and impacts (anything simpler can be solved at a glance). Toyota's problem-solving style is about learning to turn a problem with multiple causes and impacts into a one-to-one cause-and-effect problem, identifying which cause (out of all the potential factors) is responsible for the big impact (out of all the potential impacts). Developing the knowledge to correctly substitute the right cause-and-effect model for a multiple-cause/impact situation is, in effect, wisdom - and it requires hours of practice at reducing technical problems. This kind of wisdom lets you start the "five whys" analysis in the right place. If not, who knows where it will end up. The key to a successful "five whys" is a correct understanding of the current state. Without narrowing down and specifically describing the current state, asking "why" several times will probably lead you... nowhere."