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Investopedia explains 'Backtesting'
When you backtest a theory, the results achieved are highly dependent on the movements of the tested period. Backtesting a theory assumes that what happens in the past will happen in the future, and this assumption can cause potential risks for the strategy.
For example, say you want to test a strategy based on the notion that Internet IPOs outperform the overall market. If you were to test this strategy during the dotcom boom years in the late 90s, the strategy would outperform the market significantly. However, trying the same strategy after the bubble burst would result in dismal returns. As you'll frequently hear: "past performance does not necessarily guarantee future returns".
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