Saturday, December 17, 2011
MUST READ! Physics Envy
Trained as a physicist, Emanuel Derman once served as the head of quantitative analysis at Goldman Sachs and is currently a professor of industrial engineering and operations research at Columbia University. With "Models Behaving Badly" he offers a readable, even eloquent combination of personal history, philosophical musing and honest confession concerning the dangers of relying on numerical models not only on Wall Street but also in life.
Mr. Derman's particular thesis can be stated simply: Although financial models employ the mathematics and style of physics, they are fundamentally different from the models that science produces. Physical models can provide an accurate description of reality. Financial models, despite their mathematical sophistication, can at best provide a vast oversimplification of reality. In the universe of finance, the behavior of individuals determines value—and, as he says, "people change their minds."
In short, beware of physics envy. When we make models involving human beings, Mr. Derman notes, "we are trying to force the ugly stepsister's foot into Cinderella's pretty glass slipper. It doesn't fit without cutting off some of the essential parts." As the collapse of the subprime collateralized debt market in 2008 made clear, it is a terrible mistake to put too much faith in models purporting to value financial instruments. "In crises," Mr. Derman writes, "the behavior of people changes and normal models fail. While quantum electrodynamics is a genuine theory of all reality, financial models are only mediocre metaphors for a part of it."
Throughout "Models Behaving Badly," Mr. Derman treats us to vignettes from his interesting personal history, which gave him a front-row seat for more than one model's misbehavior. Growing up in Cape Town, South Africa, he witnessed the repressive and failed political model of apartheid. Later he became disillusioned with the utopian model of the kibbutz in Israel. He started out professionally in the 1970s as a theoretical physicist. He then migrated to the center of the financial world in the 1980s, using a mix of mathematics and statistics to value securities for the trading desk at Goldman Sachs in New York. He had hoped to use the methods of physics to build a grand, unified theory of security pricing. After 20 years on Wall Street, even before the meltdown, he became a disbeliever.
He sums up his key points about how to keep models from going bad by quoting excerpts from his "Financial Modeler's Manifesto" (written with Paul Wilmott), a paper he published a couple of years ago. Among its admonitions: "I will always look over my shoulder and never forget that the model is not the world"; "I will not be overly impressed with mathematics"; "I will never sacrifice reality for elegance"; "I will not give the people who use my models false comfort about their accuracy"; "I understand that my work may have enormous effects on society and the economy, many beyond my apprehension."
Sampling from models that behave well, Mr. Derman gives an eloquent description of James Clerk Maxwell's electromagnetic theory in a chapter titled "The Sublime." He writes: "The electromagnetic field is not like Maxwell's equations; it is Maxwell's equations." In another chapter, titled "The Absolute," he outlines Spinoza's "Theory of Emotions"—a description of the nature of emotions that did for man's inner life, Mr. Derman says, "what Euclid did for geometry." But then he turns to financial models—behaving badly.
The basic problem, according to Mr. Derman, is that "in physics you're playing against God, and He doesn't change His laws very often. In finance, you're playing against God's creatures." And God's creatures use "their ephemeral opinions" to value assets. Moreover, most financial models "fail to reflect the complex reality of the world around them."
It is hard to argue with this basic thesis. Nevertheless, Mr. Derman is perhaps a bit too harsh when he describes EMM—the so-called Efficient Market Model. EMM does not, as he claims, imply that prices are always correct and that price always equals value. Prices are always wrong. What EMM says is that we can never be sure if prices are too high or too low.
The Efficient Market Model does not suggest that any particular model of valuation—such as the Capital Asset Pricing Model—fully accounts for risk and uncertainty or that we should rely on it to predict security returns. EMM does not, as Mr. Derman says, "stubbornly assume that all uncertainty about the future is quantifiable."
The basic lesson of EMM is that it is very difficult—well nigh impossible—to beat the market consistently. This lesson, or "model," behaves very well when investors follow it. It says that most investors would be better off simply buying a low-cost index fund that holds all the securities in the market rather than using either quantitative models or intuition in an attempt to beat the market. The idea that significant arbitrage opportunities are unlikely to exist (and certainly do not persist) is precisely the mechanism behind the Black-Scholes option-pricing model that Mr. Derman admires as a financial model behaving pretty well.
Such a quibble aside, it is undeniable that "Models Behaving Badly" itself performs splendidly. Bringing ethics into his analysis, Mr. Derman has no patience for coddling the folly of individuals and institutions who over-rely on faulty models and then seek to escape the consequences. He laments the aftermath of the 2008 financial meltdown, when banks rebounded "to record profits and bonuses" thanks to taxpayer bailouts. If you want to benefit from the seven fat years, he writes, "you must suffer the seven lean years too, even the catastrophically lean ones. We need free markets, but we need them to be principled."
Source: WSJ - Physics Envy
“I can calculate the motions of the heavenly bodies, but not the madness of people” -Sir Issac Newton
Perhaps, time to read Mises's magnum opus - Human Action?