ELI5: Backtesting

Advanced Definition
Last updated: Jul 28, 2023

Understanding Backtesting: Testing Strategies Like a Time Traveler

Have you ever wondered how traders test their strategies before risking real money in the stock market? Well, that's where "backtesting" comes in! It might sound like a complex concept, but fear not! In this article, we'll explain backtesting in simple terms, using easy-to-understand examples to help you grasp its meaning and importance.

💡 Key Ideas

  • Time Travel for Traders: Backtesting is like pretending to be in the past to see how a trading strategy would have performed using historical market data.

  • Test Before You Invest: It allows traders to test their strategies without risking real money, helping them see if the strategy would have made money in the past.

  • Learn from the Past: Backtesting provides valuable insights about a strategy's performance and helps traders improve it before using it in real trading.

  • Not a Crystal Ball: While helpful, backtesting can't predict the future, so it's essential to combine it with other tools and strategies for successful trading.

What is Backtesting?

Backtesting is like being a time traveler in the stock market. It's a process where traders use historical market data to test their trading strategies and see how they would have performed in the past. Essentially, it lets you pretend to be in the past, making trades based on a strategy, and see how those trades would have turned out.

How Does Backtesting Work?

Imagine you have a trading strategy that tells you when to buy and sell a particular stock. Backtesting allows you to apply this strategy to historical market data and see what would have happened if you followed it in the past.

Step 1: Collecting Historical Data

The first thing you need is historical data, which is like a record of past stock prices and other relevant information. You can get this data from various sources or use specialized tools that provide it.

Step 2: Pretend Trading

With the historical data in hand, you start pretend trading. You follow your strategy's rules and "buy" and "sell" stocks based on the past data, just as if you were back in time.

Step 3: Keeping Score

As you pretend trade, you keep track of how your strategy performs. Did it make a profit? Did it lose money? By how much? You note down all these results.

Step 4: Analyzing the Results

Once you've completed the pretend trading, you analyze the results. You can see if your strategy would have made money or not, and how well it would have performed compared to just holding onto the stock.

Importance of Backtesting

Backtesting is crucial for traders because it provides valuable insights:

  1. Strategy Validation: It helps you see if your trading strategy has the potential to make money based on historical data.

  2. Risk Assessment: Backtesting lets you understand the risks associated with your strategy and how much money you could have lost.

  3. Improvement Opportunities: If your strategy didn't perform well in the backtest, you can tweak and improve it before risking real money.

  4. Confidence Boost: If your strategy performs well in the backtest, it gives you more confidence to implement it in real trading.

Limitations and Considerations

While backtesting is a valuable tool, there are some limitations to keep in mind:

  • Past Performance vs. Future Results: Just because a strategy worked well in the past doesn't guarantee it will work the same way in the future.

  • Assumptions and Biases: Backtesting relies on certain assumptions and may have biases based on the historical data used.

  • Execution and Slippage: Real trading involves execution delays and slippage, which might not be accurately reflected in backtesting.


In conclusion, backtesting is like being a time traveler in the stock market, allowing you to test your trading strategies using historical data. It's an essential step for traders to validate, improve, and gain confidence in their strategies before risking real money. However, remember that backtesting is not a crystal ball for future performance, and it's crucial to combine it with other tools and strategies for successful trading. Happy backtesting!

Additional Resources