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Five whysFive whys (or 5 whys) is an iterative interrogative technique used to explore the cause-and-effect relationships underlying a particular problem.[1] The primary goal of the technique is to determine the root cause of a defect or problem by repeating the question "why?" five times, each time directing the current "why" to the answer of the previous "why". The method asserts that the answer to the fifth "why" asked in this manner should reveal the root cause of the problem.[2] The technique was described by Taiichi Ohno at Toyota Motor Corporation. Others at Toyota and elsewhere have criticized the five whys technique for being too basic and having an artificially shallow depth as a root cause analysis tool (see § Criticism). ExampleAn example of a problem is: bolts are cross-threading in the engine block on the production line.
In this example, five iterations of asking why is sufficient to get to a root cause that can be addressed.[3] The key idea of the method 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 shelf foot, which can be immediately replaced to prevent the reoccurrence of the sequence of events that resulted in cross-threading bolts. The nature of the answer to the fifth why in the example is also an important aspect of the five why approach, because solving the immediate problem may not solve the problem in the long run; the shelf foot may fail again. The real root cause points toward a process that is not working well or does not exist.[4] In this case, the factory may need to add a process for regularly inspecting shelving units for instability, and fixing them when broken. HistoryIn history, there are early examples of repeated questions to gain knowledge, such as in Plato's Meno. Aristotle developed a different approach with the four causes to develop four fundamental types of answer to the question 'why?'. Gottfried Wilhelm Leibniz used iterative why questions in his letter to Magnus von Wedderkop in 1671, in which he applied elements of argumentation that he later used to solve the question of theodicy:
The modern 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 major 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[5] the nature of the problem as well as its solution becomes clear."[2] The tool has seen 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. 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.[6] TechniquesTwo primary techniques are used to perform a five whys analysis:[7] the fishbone (or Ishikawa) diagram and a tabular format. These tools allow for analysis to be branched in order to provide multiple root causes.[8] CriticismThe five whys technique has been criticized as a poor tool for root cause analysis. Teruyuki Minoura, former managing director of global purchasing for Toyota, criticized it as being too basic a tool to analyze root causes at the depth necessary to ensure an issue is fixed.[9] Reasons for this criticism include:
Medical professor Alan J. Card also criticized the five whys as a poor root cause analysis tool and suggested that it be abandoned because of the following reasons:[10]
To avoid these issues, Card suggested instead using other root cause analysis tools such as fishbone or lovebug diagrams.[10] See also
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