Pairs trading is a popular way to alleviate some of the risk of trading. The idea is that you find two highly correlated symbols (or two very lowly correlated symbols) and enter a position in both symbols. If the pair is highly correlated, they should move in the same direction.

Typically, an opportunity presents itself when the pair ratio breaks through a threshold that is a certain number of standard deviations away from their average standard deviation.
You would then, go long in the symbol that is under-performing and short in the symbol that is over-performing.

When the pair moves back towards its average deviation, you would then close out both positions.

Many technical analysts use the Bollinger Bands indicator to spot pairs trading opportunities. In example, Bollinger Bands are set to be 2.2 Standard Deviations away from the average.

It is important to note a number things in regards to pairs trading:
• A pairs trade is designed to be market neutral. This means that because of the positions that you take in two separate instruments, the direction of the market will not effect the position. The trade is designed to profit from the relationship between the two instruments, not the direction of the market itself.

• Correlation moves along a scale of -1 to 1 with 1 meaning the instruments are perfectly correlated. Keep in mind that pairs trades can also work with pairs that are extremely negatively correlated (close to -1). When setting up a pairs trade with negatively correlated instruments, you would want to enter the positions when the two contracts are closer together than usual with the anticipation that they will move apart in opposite directions. In this case, you would enter positions in the same direction for both instead of going long in one and short in the other.

• Another important piece of the puzzle is position size. The whole idea is to be market neutral. Therefore, you would not simply enter the same number of shares or contracts for each instrument. You would want to create the same actual dollar value in both positions. If you strictly use an equal number of shares on both sides and the dollar value of the two instruments are wildly different, then the side with the higher dollar value will have way too much weight in the trade.

• The key to pairs trading is the correlation between the two instruments. One thing that many traders fail to realize is that the correlation between instruments is ever changing. Even during the course of a trade, their correlation can change. This is why correlation is important to always monitor when in a pairs trade. The trader should be prepared to exit any trades which have drastic changes and correlation.
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