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Rob Arnott warns that most impressive backtests fail because they are "data-mined"—designed to fit historical data. His firm uses the scientific method: form a logical hypothesis first, then use data only for testing. This approach creates more robust strategies that are less likely to falter when market conditions change.
The classic scientific model involved devising a theory and then collecting data to test it. The modern paradigm, driven by big data, often reverses this. Progress now frequently comes from analyzing massive datasets first to discover patterns, and only then forming hypotheses to explain them.
When Garry Kasparov faced IBM's Deep Blue, he used "insane" opening moves to take the computer "out of the book" and away from its programming. Investors can apply this by focusing on situations where historical data is irrelevant, like spinoffs or paradigm shifts like AI's impact on power demand. This forces systematic strategies into uncharted territory where they are weakest.
To combat survivorship bias, a robust trading strategy must be continuously tested against reality. Alex Gurevich’s approach involves republishing his original book with new annotations detailing where his principles succeeded and, more importantly, where they failed, creating an 'intellectual cockpit' for readers.
Single-factor models (e.g., using only CPI data) are fragile because their inputs can break or become unreliable, as seen during government shutdowns. A robust systematic model must blend multiple data sources and have its internal components compete against each other to generate a reliable signal.
A 'thesis' is a belief to be defended, leading to confirmation bias. A 'hypothesis' is a quantitatively falsifiable statement that invites challenge. This simple linguistic shift fosters a culture of actively seeking disconfirming evidence, leading to more rational investment decisions.
Hudson River Trading shifted from handcrafted features based on human intuition to training models on raw, internet-scale market data. This emergent approach, similar to how ChatGPT is trained, has entirely overtaken traditional quant methods that relied on simpler techniques like linear regression.
When evaluating a backtest, investors should distrust any model that shows impressive returns without also revealing why the strategy is incredibly difficult to implement. A believable backtest must demonstrate the associated pain, such as long periods of underperformance or high career risk, which explains why the potential for future returns exists.
Moving from science to investing requires a critical mindset shift. Science seeks objective, repeatable truths, while investing involves making judgments about an unknowable future. Successful investors must use quantitative models as guides for judgment, not as sources of definitive answers.
To combat unreliable backtests, CFM is building "meta-models" that quantitatively predict whether a new model's results are overfitted. This systematic approach aims to replace human judgment with a data-driven process for deciding if a trading model is robust enough for production.
Data can be manipulated to tell any story after the fact. To ensure objective analysis and avoid confirmation bias, it's crucial to define your hypothesis before looking at the numbers. This prevents creating compelling but baseless narratives from random correlations.