Five whys (or 5 whys) is an iterative interrogative technique used to explore the cause-and-effect relationships underlying a particular problem. 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 "five" in the name derives from an anecdotal observation on the number of iterations needed to resolve the problem.
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.
An example of a problem is: The vehicle will not start.
- Why? – The battery is dead. (First why)
- Why? – The alternator is not functioning. (Second why)
- Why? – The alternator belt has broken. (Third why)
- Why? – The alternator belt was well beyond its useful service life and not replaced. (Fourth why)
- 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 behavior, which is indicative of reaching the root-cause level.
The last answer points to a process. This is one of the most important aspects in the five why approach – the real root cause should point toward a process that is not working well or does not exist. 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?
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 five 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." The tool has seen widespread use beyond Toyota, and is now used within Kaizen, lean manufacturing and Six Sigma.
Rules of performing a five whys analysis
In order to carry out a five whys analysis properly, the following advice should be followed:
- It is necessary to engage the management in the five whys process in the company. For the analysis itself, consider what is the right working group. Also consider bringing in a facilitator for more difficult topics.
- Use paper or whiteboard instead of computers.
- Write down the problem and make sure that all people understand it.
- Distinguish causes from symptoms.
- Pay attention to the logic of cause-and-effect relationship.
- 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 the expression "and therefore".
- Try to make our answers more precise.
- Look for the cause step by step. Don't jump to conclusions.
- Base our statements on facts and knowledge.
- Assess the process, not people.
- Never leave "human error", "worker's inattention", "blame John", etc. as the root cause.
- Foster an atmosphere of trust and sincerity.
- Ask the question "Why?" until the root cause is determined, i.e. the cause the elimination of which will prevent the error from occurring again.
- When you form the answer to the question "Why?" it should be from the customer's point of view.
While the five whys technique can be 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. Reasons for this criticism include:
- Tendency for investigators to stop at symptoms rather than going on to lower-level root causes.
- Inability to go beyond the investigator's current knowledge – the investigator cannot find causes that they do not already know.
- Lack of support to help the investigator provide the right answer to "why" questions.
- Results are not repeatable – different people using five whys come up with different causes for the same problem.
- Tendency to isolate a single root cause, whereas each question could elicit many different root causes.
These can be significant problems when the method is applied through deduction only. To avoid these issues, it is recommended that the answer to the current "why" question is verified before proceeding to the next question. 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.
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Ask why. Ask it all the time, ask it any day, and always ask it three times in a row.
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