5 Whys
The 5 Whys is a questions-asking method
Method may refer to:* Scientific method, a series of steps taken to acquire knowledge* Method , a piece of code associated with a class or object to perform a task...

 used to explore the cause/effect
Cause and effect
Cause and effect refers to the philosophical concept of causality, in which an action or event will produce a certain response to the action in the form of another event....

 relationships underlying a particular problem. Ultimately, the goal of applying the 5 Whys method is to determine a root cause
Root cause
A root cause is rarely an initiating cause of a causal chain which leads to an outcome or effect of interest. Commonly, root cause is misused to describe the depth in the causal chain where an intervention could reasonably be implemented to change performance and prevent an undesirable outcome.In...

 of a defect or problem.


The following example demonstrates the basic process:
  • The vehicle will not start. (the problem)
  1. Why? - The battery is dead. (first why)
  2. Why? - The alternator is not functioning. (second why)
  3. Why? - The alternator belt has broken. (third why)
  4. Why? - The alternator belt was well beyond its useful service life and not replaced. (fourth why)
  5. Why? - The vehicle was not maintained according to the recommended service schedule. (fifth why, a root cause)
  6. 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. (5th Why solution)
    • Purchase a different vehicle that is maintainable. (6th Why solution)

The questioning for this example could be taken further to a sixth, seventh, or even greater level. This would be legitimate, as the "five" in 5 Whys is not gospel; rather, it is postulated that five iteration
Iteration means the act of repeating a process usually with the aim of approaching a desired goal or target or result. Each repetition of the process is also called an "iteration," and the results of one iteration are used as the starting point for the next iteration.-Mathematics:Iteration in...

s of asking why is generally sufficient to get to a root cause. The real key is to encourage the trouble-shooter 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 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's interesting to note that the last answer points to a process. This is actually one of the most important aspects in the 5 Why approach...the real root cause should point toward a process. You will observe that the process is not working well or that the process does not even 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 simply asking the question why?, ask the question Why did the process fail?

Keep in mind the following key phrase as a background thought in any 5 why exercise: "People do not fail, processes do!"


The technique was originally developed by Sakichi Toyoda
Sakichi Toyoda
was a Japanese inventor and industrialist. He was born in Kosai, Shizuoka. The son of a poor carpenter, Toyoda is referred to as the "King of Japanese Inventors".- Career :...

 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
Toyota Production System
The Toyota Production System is an integrated socio-technical system, developed by Toyota, that comprises its management philosophy and practices. The TPS organizes manufacturing and logistics for the automobile manufacturer, including interaction with suppliers and customers...

. The architect of the Toyota Production System, Taiichi Ohno
Taiichi Ohno
was a prominent Japanese businessman. He is considered to be the father of the Toyota Production System, which became Lean Manufacturing in the U.S. He devised the seven wastes as part of this system. He wrote several books about the system, including Toyota Production System: Beyond Large-Scale...

, 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
, Japanese for "improvement", or "change for the better" refers to philosophy or practices that focus upon continuous improvement of processes in manufacturing, engineering, game development, and business management. It has been applied in healthcare, psychotherapy, life-coaching, government,...

, lean manufacturing
Lean manufacturing
Lean manufacturing, lean enterprise, or lean production, often simply, "Lean," is a production practice that considers the expenditure of resources for any goal other than the creation of value for the end customer to be wasteful, and thus a target for elimination...

, and Six Sigma
Six Sigma
Six Sigma is a business management strategy originally developed by Motorola, USA in 1986. , it is widely used in many sectors of industry.Six Sigma seeks to improve the quality of process outputs by identifying and removing the causes of defects and minimizing variability in manufacturing and...



There are two primary techniques used to perform 5 whys: the fishbone (or Ishikawa) diagram
Ishikawa diagram
Ishikawa diagrams are causal diagrams that show the causes of a certain event -- created by Kaoru Ishikawa . Common uses of the Ishikawa diagram are product design and quality defect prevention, to identify potential factors causing an overall effect...

, as well as a tabular format. These tools allow for analysis to be branched in order to provide multiple root causes for remedy.


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 cause
Root cause
A root cause is rarely an initiating cause of a causal chain which leads to an outcome or effect of interest. Commonly, root cause is misused to describe the depth in the causal chain where an intervention could reasonably be implemented to change performance and prevent an undesirable outcome.In...

s 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.
  • 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 as a good practice to avoid these issues.
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