I am a PhD student in Theoretical Computer Science. I started investing at the beginning for 2010 and after some hard knocks with financial stocks like Bank of America (bought at $12, dropped to $5), I started to educate myself in this area. This has become somewhat of an obsession now. I have a few basic set of rules before I invest in a business. The business should be understandable, helmed by good management, with strong balance sheet and should give me a sufficient margin of safety. I try to buy at at least 30% discount.