Stat Arb Explanation in NYT -- What Happened

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To oversimplify (sorry: you can’t explain this stuff without oversimplifying), AQR’s market neutral funds use computers to sort through a set of complex but common-sensical criteria to identify all sorts of assets — including stocks — that it believes are undervalued but gaining some momentum, which means that both price and fundamentals are improving. It buys, literally, thousands of those stocks. Then it seeks out stocks it believes are overvalued and starting to lose momentum. It shorts those stocks. What makes the fund “market neutral” is that it always tries to have the same amount long as short. Mr. Asness likes to say that it’s not really rocket science but intuitive investing; the computers mainly allow him to do it across thousands of stocks at the same time.
Mr. Asness does not suggest that he is going to be on the winning side of every trade. Not even close. Nor does Mr. Asness suggest that his strategy is risk-free. It’s not. “If you don’t take any risk, you won’t make any money,” he said. Even when things are going swimmingly, he’s going to have almost as many losing trades as winning ones. But over time the winning trades will add to better-than-average gains. In a down market, he hopes that his shorts will fall more than his longs, and in an up market, he wants the longs to rise more than the shorts.
As for risk, he adds leverage to bolster returns; indeed using borrowed money to calibrate risk is a major part of his strategy. But it’s not crazy stuff like Long-Term Capital Management, and it would be hard to argue with his results over time.
What happened in August is something that happens to every investor at times, even Warren E. Buffett: his strategy stopped working. So did Mr. Simons’s strategy and that of all the other quants. Mr. Asness’s trades weren’t just a little off — they were hugely off. The undervalued stocks he was buying were dropping steeply, but he wasn’t getting any help from the short side of his portfolio. Several “quants” I spoke to — market veterans who had been through the 1987 market crash and the 1998 Long-Term Capital disaster — told me they had never seen anything quite like it.
Why did it happen? In the immortal words of the market sage, James Grant, “On Wall Street, every good idea is driven into the ground like a tomato stake.” Quant investing, as practiced by the likes of Mr. Asness, Mr. Simons and others, has been enormously successful. And anything that’s successful on Wall Street is invariably going to be copied by others. That is exactly what’s happened in many cases at firms that did other things besides quant investing — like trading in derivatives built around subprime loans.
As these subprime instruments have cratered, investors have lost faith not just in them but in other credit derivatives. The holders of these securities had to meet margin calls and make other payments. So they had to start selling more liquid securities like, well, the kind of easily traded securities held in their quant equity portfolios, like Microsoft or I.B.M. or General Electric. And as they sold, other quant shops, like AQR, which held many of the same stocks, saw huge drops instead of small gains. Is it any wonder traders are calling this a contagion?
One line making the rounds on Wall Street is that the events of last week show that, just as with Long-Term Capital Management, the quants’ models didn’t work — that bloodless computers simply can’t anticipate events outside the norm. That line drives Mr. Asness bonkers. “In theory, what just happened is impossible, so if we stuck to the theory, we’d be dead,” he said. “We know this stuff happens.” Once they realized the magnitude, he and his partners quickly began a mild “deleveraging” to protect against even bigger losses. Eventually, AQR started buying cheap stock again — which had become even cheaper thanks to the short-term panic.
In the view of several big-time quants I spoke to, their big mistake was in not realizing that their little corner of Wall Street had become so crowded with imitators — and that when others were forced to sell, they were going to get hurt. Now they are all trying to figure out how to factor that into their thinking for the future — Mr. Asness very much included. “We have a new risk factor in our world,” he said.
So how should the rest of us feel about what just happened? Even though the worst seems to be over, I still think we should still be worried. But not because computer-driven quant funds took a tumble. That’s a symptom, not a cause. The larger issue is the contagion itself — the fact that something so out of left field, like subprime, could wind up hurting the quants.
Richard Bookstaber, a former quant manager, has recently written a book, called “A Demon of Our Own Design” (Wiley, 2007), which has become a small sensation on Wall Street. In it, he argues that the proliferation of complex financial products like derivatives, combined with use of leverage to bolster returns, will inevitably mean that there will be a regular stream of market contagions like the one we’re having now — one of which, someday, could be calamitous. To him, last week’s quant crisis is a classic case in point. “I think crises become inevitable when you have a financial structure like ours,” he said. “How deep or how frequent they are, I wouldn’t want to predict.” Well, who would?
So yes, it really is a scary world out there. But quants like Mr. Asness aren’t the reason.