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The five whys is a question asking method used to explore the cause/effect relationships underlying a particular problem. Ultimately, the goal of applying the 5 Whys method is to determine a root cause of a defect or problem. The following example demonstrates the basic process.
- My car will not start. (the problem)
- Why? The battery is dead. (first why)
- Why? The alternator is not functioning. (second why)
- Why? The alternator has broken beyond repair. (third why)
- Why? The alternator is well beyond its useful service life and has never been replaced. (fourth why)
- Why? I have not been maintaining my car according to the recommended service schedule. (fifth why, root cause)
The five iterations are not gospel; rather, it is postulated that five iterations of asking why is generally sufficient to get to a root cause. The real key is to encourage the troubleshooter to avoid assumptions and logic traps and instead to trace the chain of causality in direct increments from the effect through any layers of abstraction to the first or root cause.
The technique was originally developed by Sakichi Toyoda and was later used within Toyota Motor Corporation during the evolution of their 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." [1] The tool has seen widespread use beyond Toyota, and is now also used within Six Sigma.
Some consider the 5 Whys to be a powerful mnemonic for engineers or technically savvy individuals to help get to the true causes of problems. It has also been criticized as being too basic a tool to analyze root causes to the depth that is needed to ensure that the causes 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 - can't find causes that they don't already know
- Lack of support to help the investigator to ask the right "why" questions.
- Results aren't repeatable - different people using 5 Whys come up with different causes for the same problem.
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 as a good practice to avoid these issues. [2]