What is Backtesting? (Part I)

By Ahmad Hassam

Backtesting any trading strategy allows a trader to simulate its expected performance using historical price data. With Backtesting, traders can actually test their trading strategies and know how well they would have done if executed in the past.

Any trading strategy that does not have any ambiguity in its rules can be backtested effectively. Example of a simple trading strategy that can be backtested can be as follows.

When the MACD histogram has crossed above the zero line and the DMI+ is above DMI-, go long when the 5 period moving average has crossed above the 20 period moving averages.

Sell short when the 5 period moving averages has crossed below the 20 period moving average and the MACD histogram has crossed below the zero line and DMI- is above DMI+.

However, using the past price data to simulate future results often misleads traders into thinking that their backtested results will also give similar results in actual real time trading. This one example is just meant to illustrate that any trading strategy having clear cut rules can be backtested with the historical data.

Many potential factors can and will make hypothetical performance and actual performance differ significantly. So you should not fall into the trap of thinking that Backtesting may be a perfect method for identifying the most profitable trading strategies.

A trading strategy that may have worked very well over the past three years may work in an entirely different manner for the next three years as the market changes and evolves. One of the most important facts that you should always keep in your mind is that market change considerably overtime.

Technical indicators also need to change with the market. Do you know that often technical indicators that have been giving profitable signals in the past are subsequently unable to replicate their performance in the future? This may frustrate you. But this is exactly what makes trading a challenging endeavor.

Secondly, real time trading and trading with the past historical price data are two different things. A trading strategy in real time may be much different from the way the trading strategy behaves on Backtesting in term of trade execution. These differences can potentially skew the results.

However, you should still not underestimate the benefits of Backtesting. Backtesting can provide a trader with a reasonable expectation of the trading strategy's potential worth and usefulness. Backtesting is still the best available method for evaluating a trading strategy without actually trading it in real time environment.

Now let's discuss how to do Backtesting. Backtesting can be done by using two methods. The first one is the automated Backtesting. This is the most popular method. Automated Backtesting entails using a specialized program. The trader inputs the specific rules and criteria for the trading strategy into the Backtesting program.

Automated Backtesting is very easy. An entire picture of the past performance is created with the help of that software program. The software automatically applies those rules to the past price data and tallies the past hypothetical profits, losses and other information. - 31987

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