Fascinating Story of Quant Funds during Market Meltdown

Download 2010-1-23 The Quants: Formula for a Meltdown – WSJ.com

An older article about this topic:

Download 9-7-07 WSJ How Market Turmoil Waylaid the 'Quants'

The unusual behavior of stocks that PDT tracked had begun sometime in mid-July and had gotten worse in the first days of August. The previous Friday, about half a dozen of the biggest gainers on the Nasdaq were stocks that PDT had sold short, expecting them to decline, and several of the biggest losers were stocks PDT had bought, expecting them to rise. It was Bizarro World for quants. Up was down, down was up. The models were operating in reverse.

The market moves PDT and other quant funds started to see early that week defied logic. The fine-tuned models, the bell curves and random walks, the calibrated correlations—all the math and science that had propelled the quants to the pinnacle of Wall Street—couldn't capture what was happening.

At the time, few quants realized what was happening, but over the next few days a theory would emerge: The U.S. housing market was unraveling, leading to big losses in the mortgage portfolios of banks and hedge funds. One or more of those hedge funds needed to raise cash quickly to make up for the losses, and needed to sell assets quickly to do so. And the easiest-to-sell assets of all were stocks, those held in portfolios highly similar to quant funds across Wall Street.

The result was a catastrophic domino effect. The rapid selling scrambled the models that quants used to buy and sell stocks, forcing them to unload their own holdings. By early August, the selling had taken on a life of its own, leading to billions in losses. The meltdown also revealed dangerous links in the financial system few had previously realized—that losses in the U.S. housing market could trigger losses in huge stock portfolios that had nothing to do with housing. It was utter chaos driven by pure fear.

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