Portfolio construction and position sizing are key elements in investing. For every investor, there exists a tension between the desire to maximize the rate of growth of the portfolio while simultaneously minimizing the chance of blowing up. The Kelly Criterion is the method to determine the optimal portion of the portfolio to be invested in any given opportunity. Buffett, Munger, Whitman and Pabrai are all proponents of the theory.
John L. Kelly, Jr, the developer of the Kelly Criterion, seems to have been a remarkable character. According to his entry in Wikipedia, he was a physicist, “recreational gunslinger”, daredevil pilot, developed the vocoder, the first demonstration of which was the inspiration for the HAL 9000 computer in the film 2001: A Space Odyssey, and was a keen blackjack and roulette player, which is a little odd, because his criterion recommends against a bet on the roulette wheel. He died of a brain hemorrhage on a Manhattan sidewalk at age 41, never having used his formula to make money.
The Kelly Criterion output varies depending on two things: the investor’s certainty about the outcome of the investment (the “edge”) and the expected return (the “odds”). I have found it difficult to apply in practice. Hunter at Distressed Debt Investing has a great post on Peter Lupoff’s application of Kelly Theory to event-driven investing in Tiburon Capital Management’s portfolio. Lupoff’s post deals with some of the issues I have had, and is well worth reading.