Using moving averages in asystematic trading strategy




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Using moving averages in asystematic trading strategyUsing Moving Averages in a Systematic Trading Strategy

by Wayne A. Thorp, CFA

Wayne Thorp recently spoke at the 2015 AAII Investor Conference. For information on how to subscribe to recordings of the presentations, go to aaii/conferenceaudio for more details.

In the August issue of the AAII Journal, I covered one of the more basic technical indicators: moving averages, including how you could construct simple, weighted, and exponential moving averages, as well as some practical applications.

This article moves a step forward and examines how you can use moving averages as part of a systematic trading strategy. First, it will look at one - and two-line moving average systems and how you can use them to generate buy and sell signals, and then, it will touch upon system optimization and how traders use it to improve the profitability of a trading system.

one-line moving average systems

Although this technique is not as popular as using multiple moving averages, it will allow you to understand the concepts behind such systems without being confused by several moving averages.

When you view a single moving average in unison with a price chart, buy signals are generated when the price rises above the moving average. In a broad context, the movement of the price above a moving average is considered a bullish signal. Similarly, when the price falls below the moving average line, a sell or sell-short signal is generated. In this case, the price falling below the moving average is viewed as being bearish. Keep in mind too that the longer the time period you use to construct the moving average, the more significant the signal is.

Figure 1 illustrates a one-line moving average trading system. Here you see the price behavior of AFLAC between October 1998 and June 1999. The up and down arrows on the chart indicate where the price broke above or below the 50-day simple moving average. The up arrows indicate buy signals while the down arrows denote sell signals. Using strictly long positions—buying when the price rises above the 50-day moving average and selling when the price falls below the moving average—the three round-trip trades (buy and sell) would have yielded a total gain of over 57%. Be aware that this figure excludes commissions and does not take into account slippage. Also, it is important to point out that all of these examples assume that trades are entered and exited at the point when the moving average is “violated.” This does not mean that a trade could have been placed at that price—an issue that can have a significant impact on the return of a trade and the system as a whole.

While the AFLAC example shows that a single moving average system can be profitable, using a single moving average does have its pitfalls—whipsaws. A whipsaw occurs when a signal is generated (typically a buy), only to have the price make a sudden reversal (to the downside). By the time a sell signal is generated, the transaction results in a loss. Figure 2 illustrates a whipsaw. Here we have a price chart for Indiana Energy from April until May 1999 as well as a 50-day simple moving average. On April 12, the price broke above the moving average at $19.968 and closed at $20.75. Over a week later, on April 20, the price fell below the moving average at $19.826 and closed at $19.687. If you had followed the buy signal when the price rose above the moving average and sold when the price fell below the moving average, this trade would have resulted in a slight loss (excluding commissions, slippage, etc.). If commissions were included, the percentage loss would have been even more.

If you trade over a longer time period, whipsaws could have a noticeable impact on your profits. There are ways to safeguard yourself—such as lengthening the time period over which you calculate the moving average. In doing so, the average becomes less responsive to sudden price movements and will help eliminate whipsaws. While such tactics tend to lower the occurrence of “bad” trades, you also tend to lower the profits gleaned from “good” trades. If you lengthen the time period of the moving average, the average will react more slowly to price changes. As a result, you will be entering trades at a later time, thus perhaps missing some of the upward movement. Furthermore, you will tend to exit trades later and may sacrifice some of your gains. These are some of the trade-offs one faces when optimizing any trading system.

two-line systems

By adding additional moving averages to a trading system, you lower the risk of whipsaws or other false trading signals. When using multiple moving averages, buy and sell signals are generated at points called “crossovers”—points where two averages cross one another. The concept is similar to that used with one-line moving average systems, but with multiple averages you are watching for the lines to cross one another rather than the actual price.