The 5 Whys is a simple approach to help a team identify underlying reasons for a problem.
By repeatedly asking the question ‘Why?' (use five as a rule of thumb), you can peel away the layers of an issue, just like the layers of an onion, which can lead you to the different root causes of a problem.
The reason for a problem can often lead into another question; you may need to ask the question fewer or more than five times before you get to the origin of a problem.
The real key is to avoid assumptions and logic traps and encourage the team to keep drilling down to the real issues that underlie why different factors have aligned to contribute to something going wrong.
It is most helpful to use this tool in a group and to then share it with others to start to identify possible changes to test.