The 5 Whys is a simple tool which can help discover the cause and effect relationships underlying a problem. The method involves repeatedly asking 'why' until root causes have been determined. Though it is called the 5 Whys, you may have to ask fewer or more 'Whys'.
A simple example would be:
The vehicle will not start.
Why? - The battery is dead.
Why? - The alternator is not functioning.
Why? - The alternator belt has broken.
Why? - The alternator belt was well beyond its useful service life and not replaced.
Why? - The vehicle was not maintained according to the recommended service schedule. (root cause)
From the paper Change Resistance as the Crux of the Environmental Sustainability Problem by Jack Harich, 2010
"How do you know when to stop? A root cause has three identifying characteristics (compare to Rooney and Heuvel, 2004, who list 4 characteristics):
"1. It is clearly a (or the) major cause of the symptoms.
"2. It has no worthwhile deeper cause. This allows you to stop asking why at some appropriate point in root cause analysis. Otherwise you may find your-self digging to the other side of the planet.
"3. It can be resolved. Sometimes it’s useful to emphasize unchangeable root causes in your model for greater understanding and to avoid trying to resolve them without realizing it. These have only the first two characteristics.
"This definition allows numerous unproductive or pseudo root causes to be quickly eliminated.
"The important thing is to not stop at intermediate causes. These are plausible and easily found. Working on resolving what are in fact intermediate causes looks productive and feels productive. Intermediate cause solutions, more accurately called symptomatic solutions, may even work for awhile. But until the true root causes are resolved, powerful social agents will invariably find a way to delay, circumvent, block, weaken, or even rollback these solutions, because intermediate causes are symptoms of deeper causes. One must strike at the root."