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.
The questioning for this example could be taken further to a sixth, seventh, or higher level, but five iterations of asking why are 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.
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.
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.
Two primary techniques are used to perform a five whys analysis:
These tools allow for analysis to be branched in order to provide multiple root causes.
In order to carry out a five whys analysis properly, the following advice should be followed:
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:
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.