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The 5 why Help to identify the root causes of the problems.


The principle of 5 why is it based on the principle that one must ask the question ” why ” until we find the root cause of a problem. And it will be by solving this cause that we can eradicate.

This technique has been developed by Sakichi TOYODA and has been widely used by Toyota for decades. It is a central tool of problem solving, and Taiichi OHNO described the method of 5 why like:

« The basis of Toyota’s scientific approach…. Repeating why 5 times, the nature of the problem as well as its solution become clear

This tool has become widespread within Toyota and is now used in the process of problem solving.

A clarification is to be made about the term 5 times “why“. 5 times is only an idea order on the number of why needed to identify the root cause. In some cases it will be 3 times or 8 times why to get to the source cause.

Use and animation in the Gemba

The 5 Why is used through a problem-solving method (8D, DMAIC, PDCA…). These methods use the 5 why in their process to help identify the root cause or causes of a problem. When used, the project manager with Team1 :

  1. List the possible causes of the problem, the first level of why.
  2. Continues to ask the question why for each first level until there are no more suggestions.
  3. Identifies the cause (or causes).
  4. Identifies solutions to resolve root cause.
focus on the facts: that is to say the actions or events that actually took place. Describe them in an objective and precise way: each member of the group agrees on the formulation.
do not make a value judgement, do not interpret: Put in place durable solutions that rely on verified facts rather than behavioural.


My car won’t start2 :

  1. Why? -the battery is not charged.
  2. Why? -The alternator does not work.
  3. Why? -The alternator belt is broken.
  4. Why? -I exceeded the length recommended by the manufacturer and the belt was worn.
  5. Why? -I did not comply with the manufacturer’s recommendations (the first Cause).


My expenses are higher than expected3 :

  1. Why? -I have to pay 10% more taxes.
  2. Why? -I have not made my statement in time.
  3. Why? -I didn’t have all the necessary information about my heritage.
  4. Why? -I did not have some inherited property evaluated by expert.
  5. Why? -I did not find an expert in time. (the first Cause).


One machine stopped working4 :

  1. Why? -Because an overload has occurred and the fuses have blown.
  2. Why? -Because the lubrication of the pads was insufficient.
  3. Why? -Because the lubricating pump does not pumped enough.
  4. Why? -Because the pump shaft was damaged and vibrated.
  5. Why? -because there was no filter, which led to the inclusion of metal waste.


The automotive supplier Delphi has developed a method 5W3H5 (5 Whys, 3 Hows), which queries 5 times the problem and its successive causes, as in the examples above, but adding an additional dimension, the ” how “… The questioning of a problem becomes:

  • Why did this problem appear? (search for causes of appearance) Why? Why? …
  • Why was this problem not detected? (search for causes of non-detection) Why? Why? …
  • Why did the system put in place allow this problem to emerge? (System causes, referring to lack of anticipation and methods of prevention type FMECA) Why? Why? …


1-Japan Institute of Plant Management (1996)-TPM Encyclopedia

2- www.tuv.com

3- www.asn.fr

4-T. Ohno (1989)-The Toyota spirit

5-V. Drobinski (2013)-quality management and hidden costs

T. Ohno (1988)-Toyota production system

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