5 Whys

5 Whys is an iterative interrogative technique used to explore the cause-and-effect relationships underlying a particular problem.[1] The primary goal of the technique is to determine the root cause of a defect or problem by repeating the question "Why?" Each answer forms the basis of the next question. The "5" in the name derives from an anecdotal observation on the number of iterations needed to resolve the problem.

The technique was formally developed by Sakichi Toyoda and was used within the Toyota Motor Corporation during the evolution of its manufacturing methodologies. In other companies, it appears in other forms. Under Ricardo Semler, Semco practices "three whys" and broadens the practice to cover goal setting and decision making.[2]

Not all problems have a single root cause. If one wishes to uncover multiple root causes, the method must be repeated asking a different sequence of questions each time.

The method provides no hard and fast rules about what lines of questions to explore, or how long to continue the search for additional root causes. Thus, even when the method is closely followed, the outcome still depends upon the knowledge and persistence of the people involved.


  1. Why? - The battery is dead. (First why)
  2. Why? - The alternator is not functioning. (Second why)
  3. Why? - The alternator belt has broken. (Third why)
  4. Why? - The alternator belt was well beyond its useful service life and not replaced. (Fourth why)
  5. Why? - The vehicle was not maintained according to the recommended service schedule. (Fifth why, a root cause)

The questioning for this example could be taken further to a sixth, seventh, or higher level, but five iterations of asking why is generally sufficient to get to a root cause. The key is to encourage the trouble-shooter to avoid assumptions and logic traps and instead trace the chain of causality in direct increments from the effect through any layers of abstraction to a root cause that still has some connection to the original problem. Note that, in this example, the fifth why suggests a broken process or an alterable behaviour, which is indicative of reaching the root-cause level.

It is interesting to note that the last answer points to a process. This is one of the most important aspects in the 5 Why approach - the real root cause should point toward a process that is not working well or does not exist.[3] Untrained facilitators will often observe that answers seem to point towards classical answers such as not enough time, not enough investments, or not enough manpower. These answers may be true, but they are out of our control. Therefore, instead of asking the question why?, ask why did the process fail?

A key phrase to keep in mind in any 5 Why exercise is "people do not fail, processes do".


The technique was originally developed by Sakichi Toyoda and was used within the Toyota Motor Corporation during the evolution of its manufacturing methodologies. It is a critical component of problem-solving training, delivered as part of the induction into the Toyota Production System. The architect of the Toyota Production System, Taiichi Ohno, described the 5 Whys method as "the basis of Toyota's scientific approach . . . by repeating why five times, the nature of the problem as well as its solution becomes clear."[4] The tool has seen widespread use beyond Toyota, and is now used within Kaizen, lean manufacturing, and Six Sigma.


Two primary techniques are used to perform a 5 Whys analysis:[5]

These tools allow for analysis to be branched in order to provide multiple root causes.[6]

Rules of performing 5Whys

In order to carry out the 5-Why analysis properly, the following advice should be followed:

  1. It is necessary to engage the management in 5Whys standard in the company. For the analysis itself, remember about right working group. Also consider facilitator presence for more difficult topics.
  2. Let’s use paper or whiteboard instead of computers.
  3. Let’s write down the problem and make sure that all people understand it.
  4. Let’s distinguish causes from symptoms.
  5. Let’s take care of the logic of cause-and-effect relationship.
  6. Let’s make sure that root causes certainly lead to the mistake by reversing the sentences created as a result of the analysis with the use of expression “and therefore”.
  7. Let’s try to make our answers more precise.
  8. Let’s look for the cause step by step. Don’t jump to conclusions.
  9. Let’s base on facts and knowledge.
  10. Let’s assess the process, not people.
  11. Never leave “human error”, “worker’s inattention”, etc. as the root cause.
  12. Let’s take care of the atmosphere of trust and sincerity.
  13. Let’s ask the question “why” until the root cause is determined, i.e. such cause the elimination of which will cause that the error will not occur again.[7]


While the 5 Whys is a powerful tool for engineers or technically savvy individuals to help get to the true causes of problems, it has been criticized by Teruyuki Minoura, former managing director of global purchasing for Toyota, as being too basic a tool to analyze root causes to the depth that is needed to ensure that they are fixed.[8] Reasons for this criticism include:

These can be significant problems when the method is applied through deduction only. On-the-spot verification of the answer to the current "why" question before proceeding to the next is recommended to avoid these issues. In addition, performing logical tests for necessity and sufficiency at each level can help avoid the selection of spurious causes and promote the consideration of multiple root causes.[9]

See also


  1. "Five Whys Technique". adb.org. Asian Development Bank. February 2009. Retrieved 26 March 2012.
  2. Semler, Ricardo (2004). The Seven-Day Weekend. Penguin. ISBN 9781101216200. Ask why. Ask it all the time, ask it any day, and always ask it three times in a row.
  3. Ivan Fantin (2014). Applied Problem Solving. Method, Applications, Root Causes, Countermeasures, Poka-Yoke and A3. How to make things happen to solve problems. Milan, Italy: Createspace, an Amazon company. ISBN 978-1499122282
  4. Taiichi Ohno; foreword by Norman Bodek (1988). Toyota production system: beyond large-scale production. Portland, Or: Productivity Press. ISBN 0-915299-14-3.
  5. "An Introduction to 5-why". Retrieved 2010-03-06.
  6. "5-why Analysis using an Excel Spreadsheet Table". Retrieved 2010-12-25.
  7. Brożyńska, Martyna; Kowal, Krzysztof; Lis, Anna; Szymczak, Michał (2016). 5xWhys. Method First Handbook. Łódź, Poland: 2K Consulting. p. 34. ISBN 978-83-939309-2-0.
  8. "The "Thinking" Production System: TPS as a winning strategy for developing people in the global manufacturing environment". Retrieved 2014-02-19.
  9. Wilson, Bill. "Five-by-Five Whys". http://www.bill-wilson.net. Retrieved 7 October 2014. External link in |website= (help)
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