Five Whys

"... asking why five times.

When something goes wrong, we tend to see it as a crisis and seek to blame. A better way is to see it as a learning opportunity. Not in the existential sense of general self-improvement. Instead, we can use the technique of asking why five times to get to the root cause of the problem."

http://startuplessonslearned.blogspot.com/2008/11/five-whys.html