
Pairs trading often involves heavy doses of leverage because spreads of
highly correlated assets might only need to get a tiny bit out of whack to
make the trade worthwhile. Leverage in these situations is where many
hedge funds get in trouble. In 99 times out of 100, the spread might get back
in shape, but if just one or two get way out of line, then that could be the dif-
ference between a 20 percent return on the year and a –50 percent return
on the year.
The other problem with pairs trading in general is that there are many
billions of dollars in hedge funds doing a variety of pairs trading strategies,
merger arbitrage being the most common pairs trading strategy. If Com-
pany A is buying Company B in stock, then suddenly those two assets
should trade in perfect lockstep with each other, and as the deal date gets
close, the price of Company B should converge on the price that Company
A is paying for it. Because the amount of money involved in playing the
merger arbitrage strategy is so great, the only way to really lock in a good
return in most cases (particularly when the deal is extremely likely to
close) is to use leverage. If the deal falls through at the last minute, as oc-
casionally happens, look out below.
There are other types of pairs trading strategies, for instance, going
long a basket of low P/E stocks in a sector and short high P/E stocks in the
same sector. Going long and short same duration bonds that expire on dif-
ferent dates (the closer the date, the more likely it is to be accurately
priced, whereas the further out bond might be mispriced). The most com-
mon type of pairs trading strategy, which we focus on in this technique, is
when the ratio (spread) between two assets that are highly correlated
moves out of kilter with its historical norm.
The key difference in using unilateral pairs trading is that we will
not trade both sides of the spread, but the side that is historically more
volatile. The idea is that the more volatile side is most often the culprit for
why the spread has gone awry. For this reason we will treat QQQ and SPY
as a pair, but we will only trade QQQ.
THE UNILATERAL PAIRS TRADING
SYSTEM FOR QQQ-SPY
1. Calculate the ratio of the QQQ price series over the SPY price series.
For example, on May 1, 2003, SPY was 91.92 and QQQ was 27.42. The
ratio for that day was 27.42/91.92, or 0.298.
2. Calculate the 20-day moving average of that ratio.
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