The 5 whys method. Define the problem and keep asking why until you find the real reason for the problem.

1. The 5 Whys

2. Step # 1
Describe what the problem is.
Step # 2
Ask why several times to better understand the reasons
for the problem. Keep asking why until you get to the real
reason for the problem.

3. Sources
http://www.bulsuk.com/2009/03/5-why-finding-root-causes.html
http://blog.gembaacademy.com/2010/01/10/introducing-the-5-why-%E2%80%9Cso-what%E2%80%9D-test/
https://hbr.org/video/2189146765001/5-whys
http://www.ideachampions.com/weblogs/archives/2011/05/some_years_ago.shtml
http://www.lifehack.org/572725/why-critical-thinking-essential-your-success-and-how-you-can-improve
http://www.mindtools.com/pages/article/newTMC_5W.htm
http://thinkjarcollective.com/tools/how-to-formulate-a-creative-problem-statement/
http://youtu.be/JmrAkHafwHI
http://en.wikipedia.org/wiki/5_Whys