The 5 Whys is an iterative question-asking 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. (The "5" in the name derives from an empirical observation on the number of iterations typically required to resolve the problem.)
The following example demonstrates the basic process:
- The vehicle will not start. (the problem).
- 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)
- Why? - Replacement parts are not available because of the extreme age of the vehicle. (sixth why, optional footnote)
- Start maintaining the vehicle according to the recommended service schedule. (possible 5th Why solution)
- Purchase a different vehicle that is maintainable. (possible 6th Why solution)
The questioning for this example could be taken further to a sixth, seventh, or higher level: the "five" in 5 Whys is not gospel, 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 typical 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. 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 sometimes be true but in most cases they lead to answers 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." The tool has seen widespread use beyond Toyota, and is now used within Kaizen, lean manufacturing, and Six Sigma.
There are two primary techniques used to perform 5 Whys: the fishbone (or Ishikawa) diagram and a tabular format. These tools allow for analysis to be branched in order to provide multiple root causes.
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. 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 - cannot find causes that they do not already know.
- Lack of support to help the investigator ask the right "why" questions.
- Results are not repeatable - different people using 5 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. On-the-spot verification of the answer to the current "why" question before proceeding to the next is recommended to avoid these issues.
See also 
- "Five Whys Technique". adb.org. Asian Development Bank. February 2009. Retrieved 26 March 2012.
- Taiichi Ohno; foreword by Norman Bodek (1988). Toyota production system: beyond large-scale production. Portland, Or: Productivity Press. ISBN 0-915299-14-3.
- "An Introduction to 5-why". Retrieved 6 March 2010.
- "5-why Analysis using an Excel Spreadsheet Table". Retrieved 25 December 2010.
- "The "Thinking" Production System: TPS as a winning strategy for developing people in the global manufacturing environment". Retrieved 2011-02-02.