A mathematical hard line at the number two determines whether a trading strategy will succeed or fundamentally break.
April 25, 2026
Original Paper
The Matched Filter Principle for Trading Strategy Regime Detection: Tail Index as the Universal Threshold Between Filter Designs
SSRN · 6621520
The Takeaway
The tail index of a market's return distribution dictates which filtering tools actually work. Strategies that rely on rolling averages fail completely once they cross this specific threshold. This explains why certain quant models work for years and then suddenly collapse. Traders have been using the wrong tools for fat-tailed markets without realizing the math was against them. Identifying this threshold allows for the design of systems that are truly resilient to market shifts.
From the abstract
The optimal regime detection filter for a trading strategy must match the distributional structure of the strategy's return sequence. Applying a mismatched filter produces degenerate or spurious regime signals; applying a matched filter extracts maximum information from the performance history. This paper formalises the Matched Filter Principle by connecting the signal processing matched filter theorem to the Neyman-Pearson framework for regime detection in trade outcome sequences. The tail inde