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Wednesday, March 11, 2009
Posted by Roberto
10:05 AM 0 comments

DO "QUANTS" ADD UP ON WALL STREET?

What follows is an excerpt from the article written by Dennis Overbye and published on page 12 of the Global Edition of the New York Times on Wednesday, March 11, 2009.

They are known as "quants" because they do quantitative finance. Seduced by a vision of mathematical elegance underlying some of the messiest of human activities, they apply skills they once hoped to use to untangle string theory or the nervous system to making money. Quants occupy a revealing niche in modern capitalism. They make a lot of money but not as much as the traders who tease them and treat them like geeks. Until recently they rarely made partner at places like Goldman Sachs. In some quarters they get blamed for the current breakdown - "All I can say is, beware of geeks bearing formulas," Warren Buffett said on "The Charlie Rose Show" last fall. Even the quants tend to agree that what they do is not quite science.

As Dr. Derman put it in his book "My Life as a Quant: Reflections on Physics and Finance", "In physics there may one day be a Theory of Everything; in finance and the social sciences, you're lucky if there is a useable theory of anything." In the 1970s the late Fischer Black of Goldman Sachs, Myron S. Scholes of Stanford and Robert C. Merton of Harvard had figured out how to price and hedge these options in a way that seemed to guarantee profits. The so-called Black-Scholes model has been the quants' gold standard ever since. In the old days, Dr. Derman explained, if you thought a stock was going to go up, an option was a good deal. But with Black-Scholes, it doesn't matter where the stock is going. Assuming that the price of the stock fluctuates randomly from day to day, the model provides a prescription for you to still win by buying and selling the underlying stock and its bonds. The Black-Scholes equation resembles the kinds of differential equations physicists use to represent heat diffusion and other random processes in nature. Except, instead of molecules or atoms bouncing around randomly, it is the price of the underlying stock. The price of a stock option, Dr. Derman explained, can be interpreted as a prediction by the market about how much bounce, or volatility, stock prices will have in the future. But it gets more complicated than that. For example, markets are not perfectly efficient - prices do not always adjust to right level and people are not perfectly rational. Indeed, Dr. Derman told The Times, the idea of a "right level" is "a bit of a fiction." As a result, prices do not fluctuate according to Brownian motion. Rather, he said: "Markets tend to drift upward or cascade down. You get slow rises and dramatic falls."

One consequence of this is something called the "volatility smile," in which options that benefit from market drops cost more than options that benefit from market rises.
Another consequence is that when you need financial models the most - on days like Black Monday in 1987 when the Dow dropped 20 percent - they might break down.

One of the most outspoken critics is Nassim Nicholas Taleb, a former trader and now a professor at New York University. He got a rock-star reception at the World Economic Forum in Davos this winter. In his best-selling book "The Black Swan" (Random House, 2007), Dr. Taleb, who made a fortune trading currency on Black Monday, argues that finance and history are dominated by rare and unpredictable events.

"Every trader will tell you that every risk manager is a fraud," he told The Times, and options traders used to get along fine before Black-Scholes. "We never had any respect for nerds."

Dr. Taleb has waged war against one element of modern economics in particular: the assumption that price fluctuations follow the familiar bell curve that describes, say, IQ scores or heights in a population, with a mean change and increasingly rare chances of larger or smaller ones, according to so-called Gaussian statistics named for the German mathematician Friedrich Gauss.

But many systems in nature, and finance, appear to be better described by the fractal statistics popularized by Benoit Mandelbrot of I.B.M., which look the same at every scale. An example is the 80-20 rule that 20 percent of the people do 80 percent of the work, or have 80 percent of the money. Within the blessed 20 percent the same rule applies, and so on. As a result the odds of game-changing outliers like Bill Gates's fortune or a Black Monday are actually much greater than the quant models predict, rendering quants useless or even dangerous, Dr. Taleb said.

Links:
They Tried to Outsmart Wall Street
The Black-Scholes model