A deep understanding of the fundamentals so that pricing inefficiencies can be identified.
Adding value (getting a return greater than that available from passive investing) requires one to see how markets are mispriced, and this requires an understanding of how they should be priced. This is required to be a winner over time. It is the equivalent of being able to count cards and calculate the odds of a winning hand in poker - it is the fundamental assessment that allows you to discern a good bet from a bad one.
Some people say that understanding the fundamentals isn't required and that one can play and win the game by playing it technically. If by technical they mean an approach that is devoid of understanding fundamental cause-effect relationships - like trend following - then I believe that they are wrong. Sometimes markets trend, and sometimes they chop, and they do so for reasons. So, without an understanding of these reasons, one will be blindly betting that markets trend more than they chop. Do markets trend more than they chop? This is one of those cosmic questions that can't be definitely answered, and certainly not without an understanding of the fundamentals that determine market behavior.
There is no escaping the need to have a deep understanding of the fundamentals so that one can sensibly assess what is cheap and what is expensive. In playing poker I would rather place my bet based on my ability to count the cards and calculate odds than on the likelihood of a hot streak continuing (e.g. betting that I will do well because I won the last few hands).
Adding value is a zero-sum game - for me to add value I must be a better player than my opponents. The markets are extremely competitive. That means that my understanding must be very deep, which requires focus. I have rarely seen investors that win over time who trade a lot of different markets. The winners I know discuss their markets with the same depth that specialists in other professions (e.g. physicians, scientists, etc.) discuss the subjects of their focus.
In addition, successful market players have the capacity to think conceptually and independently. Equipped with knowledge and perspective, they can justifiably have the confidence to stand apart from the crowd, which is essential for being able to buy low and sell high.
Perspective without data-mining.
Many years ago I did a lot of discretionary trading based on the flow of information I was seeing at the time. I wrote down the criteria I used to make each trade so that I could reflect on the trade later. I learned that if I specified the criteria clearly I could see how these criteria would have worked in the past, and in different countries, which gave me perspective. That perspective was invaluable.
In many cases I learned that the criteria wouldn't have worked in the past and I could see why. In other cases I learned how well my decision rule worked so that I would not abandon it when it lost (all rules lose sometime) or put too much on it because it has recently been hot and I thought it was better than it really was. As a result, I developed a good sense of what I could expect from my criteria.
I learned that I could program the computer to scan the world for opportunities, according to these criteria. And I learned a lot more. I learned to be especially wary about data mining - to not go looking for what would have worked in the past, which will lead me to have an incorrect perspective. Having a sound fundamental basis for making a trade, and an excellent perspective concerning what to expect from that trade, are the building blocks that have to be combined into a strategy.Taken from The Global-Investor Book of Investing Rules