The 5 Whys is a question asking technique used to determine the root cause of a problem.
The 5 in the title suggests it should take no more than 5 questions to get to the root cause of the problem.
Developed by the founder of Toyota Sakichi Toyoda, the 5 Whys Technique is used in problem solving, trouble shooting and improving processes.
It may not solve the problem by itself but may guide you to an alternative path to follow, for example using a cause and effect diagram (Ishikawa diagram) or other problem solving tool to fix the problem.
The technique is designed to guide you to the root of the problem.
Here's how to use the 5 Whys Technique.
Start with the problem and ask a 'why' question about the problem. The next 'why' question you ask should then follow on from the answer to the first question. Here's an example:
The vehicle will not start. (the problem)
You could possibly take this to a 6th or 7th why, but 5 is usually enough to get to the root of the problem.
It's important to understand that typically the 5th why doesn't point to a solution - it points to processes. This answer doesn't tell you how to fix the problem, only what caused it.
This is where some other problem solving tool will come in useful to determine why the root cause existed. So in the above example, the process of servicing the vehicle failed and this is then what needs to be corrected. A cause and effect diagram can then help you to determine which part of the process failed and what corrective actions need to be taken to correct this for future.
There has been some criticism of the tool in that its too basic or doesn't fix the problem. But when time is short and you need to get to the root cause quickly then the technique is incredibly useful.
Example taken from Wikipedia