Quote from inflector:
Anyone who says that testing using historical price data has no value is not thinking clearly.
How does one arrive at any strategy without knowledge of the past? How do you determine when to buy or sell?
What are the alternatives? Guessing?
All we have is what the markets have done so far. Even if we are trading live as a discretionary trader using only our personal experience as a guide for what to do in the future, we are relying on subjective back-testing.
It's not hard to argue that computers can more reliably test ideas using the same data. Backtesting works for the same reasons that smart traders can develop systems after years of experience in trading. They note repeating patterns that offer opportunities for profits. They then trade using ideas designed to capitalize on those opportunities.
You can do the same thing with a computer but with a more rigorous analysis. You will often find that ideas that appeared promising won't work because of something that you did not anticipate. It is much better to find this out using a computer than an actual account.
The reason that some people shy away from backtesting is that there are many ways to screw up backtesting and to find methods that appear to work that won't actually work in real markets. These problems are tractable if you pay attention to avoid the most common pitfall: over optimization.
Just because someone can use a knife to slit their wrists doesn't mean you don't want to use one in the kitchen when cooking. You just need to be careful with sharp instruments.
- Curtis
Before you open your mouth Mr. Turtle post your trading performance over the last2 years!

