It’s a sobering reminder of how spiffy new computer models and mathematical formulas related to real-world events can end up as just the old “garbage in, garbage out,” because we don’t know all the variables to include. In the case of something called “Li’s cupola function,” a beautiful mind came up with a beautiful mathematical formula that was applied by investors to mortgage risk, and it ended up spelling economic doom for most of us. Oops!:
It was a brilliant simplification of an intractable problem. And Li didn’t just radically dumb down the difficulty of working out correlations; he decided not to even bother trying to map and calculate all the nearly infinite relationships between the various loans that made up a pool. What happens when the number of pool members increases or when you mix negative correlations with positive ones? Never mind all that, he said. The only thing that matters is the final correlation number—one clean, simple, all-sufficient figure that sums up everything.
A few people warned that the map was not the territory, but their cautions were ignored, and why? Too many people were making too much money, that’s why. At least for a while:
Banks dismissed [warnings], partly because the managers empowered to apply the brakes didn’t understand the arguments between various arms of the quant universe. Besides, they were making too much money to stop.
Here’s the mind-boggling part, to my way of thinking. The following should have been a red flag the size of Texas:
[B]ecause the copula function used CDS prices to calculate correlation, it was forced to confine itself to looking at the period of time when those credit default swaps had been in existence: less than a decade, a period when house prices soared. Naturally, default correlations were very low in those years. But when the mortgage boom ended abruptly and home values started falling across the country, correlations soared.
Don’t blame Li—he just made the model, he didn’t apply it. Those who did were unaware of its limitations, partly because they didn’t get the math, partly because they decided to ignore history, and partly because there was gold in them thar hills.