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?" five times. The answer to the fifth why should reveal the root cause of the problem.
An example of a problem is: the vehicle will not start.
- Why? – The battery is dead.
- Why? – The alternator is not functioning.
- Why? – The alternator belt has broken.
- Why? – The alternator belt was well beyond its useful service life and not replaced.
- Why? – The vehicle was not maintained according to the recommended service schedule. (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 troubleshooter 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. 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 resources. These answers may be true, but they are out of our control. Therefore, instead of asking 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, lean construction and Six Sigma. The five whys were initially developed to understand why new product features or manufacturing techniques were needed, and was not developed for root cause analysis.
These tools allow for analysis to be branched in order to provide multiple root causes.
The five whys have been criticized as a poor tool for root cause analysis. Teruyuki Minoura, former managing director of global purchasing for Toyota, criticized them 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.
Medical professor Alan J. Card also criticized the five whys as a poor root cause analysis tool and suggested that it be abandoned entirely. His reasoning also includes:
- The artificial depth of the fifth why is unlikely to correlate with the root cause.
- The five whys is based on a misguided reuse of a strategy to understand why new features should be added to products, not a root cause analysis.
- Eight disciplines problem solving
- Five Ws (information-gathering)
- Four causes
- Issue map
- Issue tree
- Root cause analysis
- Socratic method
- Why–because analysis
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- Fantin, Ivan (2014). Applied Problem Solving. Method, Applications, Root Causes, Countermeasures, Poka-Yoke and A3. ISBN 978-1499122282.
- Ohno, Taiichi (March 2006). ""Ask 'why' five times about every matter."". Archived from the original on Nov 27, 2022. Retrieved September 5, 2019.
- 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.
- Bulsuk, Karn (April 2, 2009). "An Introduction to 5-why". Retrieved September 5, 2019.
- Bulsuk, Karn (July 7, 2009). "5-whys Analysis using an Excel Spreadsheet Table". Retrieved September 5, 2019.
- "The "Thinking" Production System: TPS as a winning strategy for developing people in the global manufacturing environment" (PDF). Public Affairs Division, Toyota Motor Corporation. October 8, 2003. Archived from the original (PDF) on November 21, 2020. Retrieved September 5, 2019.
- Card, Alan J. (August 2017). "The problem with '5 whys'". BMJ Quality & Safety. 26 (8): 671–677. doi:10.1136/bmjqs-2016-005849. PMID 27590189. S2CID 42544432.