Keep it Simple!

I have been convinced, in my investment story, that it was not necessary a deep analysis to buy a stock in the market. You should look for general indicators, like profitability, margins, competition, market multiples (P/E, Yield) and so on, and start to buy when the asset reaches a nice price, keeping the amount low to the hole portfolio (my portfolio usually doesn't have more than 8-10% of one stock). 

Trying to research everything is, at least, impossible. And it's counter-productive, you have to apply to much time and there's no way to know if you're not wrong, or if something could not goes wrong.

Well, why I'm saying that? Because yesterday I was reading an interview of the master Graham and realized that this might be the correct way:

In selecting the common stock portfolio, do you advise careful study of and selectivity among different issues? In general, no. I am no longer an advocate of elaborate techniques of security analysis in order to find superior value opportunities. This was a rewarding activity, say, 40 years ago, when our textbook "Graham and Dodd" was first published; but the situation has changed a great deal since then. In the old days any well-trained security analyst could do a good professional job of selecting undervalued issues through detailed studies; but in the light of the enormous amount of research now being carried on, I doubt whether in most cases such extensive efforts will generate sufficiently superior selections to justify their cost. To that very limited extent I'm on the side of the "efficient market" school of thought now generally accepted by the professors.

What general approach to portfolio formation do you advocate? Essentially, a highly simplified one that applies a single criteria or perhaps two criteria to the price to assure that full value is present and that relies for its results on the performance of the portfolio as a whole--i.e., on the group results--rather than on the expectations for individual issues.

It's not much like Buffett would say, but I'm now convinced that it's the core principle of Graham: have a diversified portfolio, bought at a low price. If something goes wrong with a determined stock, you don't broke up. And your performance will be determined considering the whole portfolio (that's supposed to be cheap, with a high margin of safety).