The "5 Why" method
The 5 Why method is a way of conducting incident analysis which is originally developed in the 70’s by Sakichi Toyoda and was later used within Toyota Motor Corporation during the evolution of their manufacturing methodologies. It is a simple but effective method to find the cause of incidents.
The 5 Why method is a question-asking method that is used to understand the cause/consequence relationships that underlie a particular problem. The ultimate goal of applying the 5 Whys method, is to determine a root cause of an event or problem. The idea is to ask the question why the event happened and to ask why for that answer as well until you reach the root cause of the event. Originally the method prescribes that five iterations of asking why is generally sufficient to get to a root cause. But nowadays a sixth, seventh or even greater level is used as well. The purpose remains to find the root cause to the original event through any amount of levels of abstraction and to encourage the user to avoid assumptions and logic traps. The answer to the last question, or the root cause should always be an organizational factor on a systemic process level. To reach this level it is advisable to ask the question ‘Why did the process fail?’ instead of asking the question ‘Why?’ when the fifth level is reached. The background thought in the 5 why method is: "People do not fail, processes do!". This method is closely related to the Cause & Effect (Fishbone) diagram.