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Suppose that over time traders have experimented with trading rules, drawn from a very wide universe of trading rules, perhaps tens of thousands of different iterations. As time progresses the rules that happen to perform well historically, attract more attention and are considered serious rules by the trading community, while unsuccessful rules gradually fall by the wayside.

If enough trading rules are considered over time, some rules, by pure luck, even in a large sample, will produce superior performance, even if they do not genuinely possess predictive power.

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