I look for opportunities to invest where the expected value is sufficiently greater than the cost to invest and look to invest the appropriate portion of the total funds available. To make a gambling analogy, a highly favorable investment would be one where you could invest $1 on a flip of a coin and receive $10 if it flipped heads and lose only $1 if it came up tails. However, you would not want to invest all of your funds because you would be broke if the coin turned up tails. Thus, the goal is to find investments where the edge is sufficiently large and then invest the appropriate portion of the funds. The Kelly formula provides a theoretical basis for the appropriate percentage of total funds to invest in a single opportunity.
However, in the real world, the precise odds are rarely known. Thus, I seek to develop the ability and obtain the knowledge to calculate the odds with a degree of accuracy, and conservatively enough, to be able to make intelligent invesments.
With regards to equities, which I have primarily invested in, I seek to understand the economics of the business so as to evaluate its potential for long term success or failure. I seek to use this understanding, along with an examination of its financial statements, to determine if the company is undervalued or overvalued. I may then decide to go long undervalued companies and I may decide to short overvalued companies. My preference is to find companies to purchase, rather than to short.