Where the method comes from

Taiichi Ohno, the architect of the Toyota Production System in the 1950s, describes in his book "Toyota Production System: Beyond Large-Scale Production":

In a production plant, data is highly valued - but I think facts are even more important. If we don't look thoroughly for the cause of a problem when it occurs, the measures taken can become out of focus. That's why we keep asking: why? This is the scientific basis of the Toyota system. [...] For every problem I always ask "why" five times?

This is central

"Quick fixes" may seem appropriate for simple problems, but often only solve the visible part of the problem. With the quick 5-Why or 5-W method, deeper symptoms can be identified.

This is the procedure

The first step is to describe the problem precisely and concisely by ensuring that it is understood equally by all.
By asking five questions beginning with "Why", we try to find the most plausible reasons for the problem. For each answer, we ask a new question starting with "Why".
Make sure that the identified causes lead to the problem being investigated by using the phrase "and therefore... " is answered.
Do not evaluate the people but the process. Do not conclude the analysis with "human error", "employee inattention" as the cause. The local system should be strong enough to minimize the human factor (fatigue, inattention, etc.).

This method is also used to construct the Ishikawa diagram and can identify several causes.

Example from the book by Taiichi Ohno:
"Suppose a machine stops working:
1. Why did the machine stop?
There was an overload and the fuse blew.
2. Why was there an overload?
The bearing was not sufficiently lubricated.
3. Why was not enough lubrication?
The lubrication pump was not pumping sufficiently.
4. Why was there insufficient pumping?
The pump shaft was worn and rattled.
5. Why was the shaft worn?
No pickling agent was applied and metal scrap penetrated.

Repeating the question five times can help to uncover and correct the basic problem. If this procedure were not performed, the fuse or pump shaft could simply be replaced. In this case the problem would reappear within a few months.

Our perspective

If the problem assessment is based solely on intuition, there is a danger of not identifying the true causes of the problem. It is therefore important to include verifiable facts and to involve the experts.
There is always a risk that the results of this method are not reproducible, because they depend on the team and can lead to different causes. It is therefore important to strongly recommend checking each answer with facts.

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