Marc Gerstein, a Forbes author who was until very recently Portfolio123’s director of research (you can see his work in stock models he created like…
As Michael Mauboussin relates, not too long ago the Columbia Business School sent a group of students to meet with Todd Combs, the investment manager at Berkshire Hathaway and (currently) CEO of Geico. He recommended that they read 500 pages a day. The students were dumbfounded. Combs’s colleague at Berkshire, Vice Chairman Charlie Munger, has said, “In my whole life, I have known no wise people (over a broad subject matter area) who didn’t read all the time—none, zero.” And Warren Buffett himself has suggested that he devotes 80% of his working day to reading.
Mauboussin writes, “success in investing has two parts: finding edge and fully taking advantage of it through proper position sizing. Almost all investment firms focus on edge, while position sizing generally gets much less attention.” This is because position sizing is a forbidding concept. If you try mean-variance portfolio optimization or using the Kelly criterion to decide how much to put into each stock you own, you’re likely to get bogged down in remarkably complex computations with results that are indefinite at best.
Mauboussin breaks down two functions of the price of a stock. First, it tells us (gives us information about) how much the market believes a stock is worth. Second, it acts as an influence upon buyers: if a price is rising, people want to get in on the rise and buy; if a price is falling, investors are more likely to want to sell. The task of a great investor is to learn how to separate the two, subscribe only to the information, and ignore the influence.
Let’s say two investors buy the same stock. Investor A paid $70 for it and investor B pail $100 for it. The stock is presently worth $85. Investor A will be happy and likely to think favorably of the stock because he made 21% on it; investor B will be sad and likely to think unfavorably of the stock because he lost 15% on it.
The other night I asked my son, a high school student who knows next to nothing about economics or the stock market, “Which is more likely to grow faster, a small company or a large company?” He answered, “A small company.” Then I asked him, “Which will rise in price faster, a company whose price is cheap compared to what it earns, or a company whose price is expensive?” He answered, “The cheap one.”
Mauboussin writes, “Great investors do two things that most of us do not. They seek information or views that are different than their own and they update their beliefs when the evidence suggests they should. Neither task is easy. . . . The best investors among us recognize that the world changes constantly and that all of the views that we hold are tenuous. They actively seek varied points of view and update their beliefs as new information dictates. The consequence of updated views can be action: changing a portfolio stance or weightings within a portfolio.”
Whenever you come up with a new investment idea—whether it’s a new security to buy, a new factor to consider, or a new strategy to implement—you naturally ask yourself whether this new idea will increase your portfolio returns or cause you to lose money (and, of course, how much). Thinking probabilistically involves assessing the probabilities and coming up with a reasoned answer.