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?”. Each answer forms the basis of the next question.
The “five” in the name derives from an anecdotal observation on the number of iterations needed to resolve the problem.
An example of a problem is: The vehicle will not start.
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)
Author = Wikipedia