The factor-residual model at the heart of stat-arb decomposes each stock's return into a systematic factor component and an idiosyncratic residual. The S-score is a trading signal on the idiosyncratic residual. But a portfolio of long-short positions on residuals is not automatically free of systematic factor exposure — construction errors, estimation errors, and portfolio-level correlations can leave residual beta, sector, or leverage-factor risk. These unintended exposures are what the portfolio risk process monitors and eliminates.
The most dangerous version of this problem played out in August 2007: hundreds of quant funds were exposed to the same hidden factor (correlated "value" signals), and a single large forced liquidation triggered cascading margin calls across the industry. The quant quake was a risk management failure, not a signal failure — the signals were correct, but the common-investor risk was not hedged.
Learning objectives. By the end of this note you should be able to:
- define systematic and unsystematic risk in the context of a stat-arb book and identify what the risk model controls;
- describe beta hedging and sector neutralisation as the two primary factor hedging operations;
- specify a drawdown-based leverage circuit-breaker and explain the VaR-targeting feedback loop;
- state the four drivers of the August 2007 quant quake and explain why they amplified each other;
- explain contagion (common-investor) risk and contrast it with conventional market risk.
In this lesson
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- Types of risk in a stat-arb portfolio
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- Factor hedging
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- VaR targeting and leverage control
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- Drawdown-based circuit breakers
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- The quant quake of August 2007
Before you start
- Portfolio Construction: Rule-Based vs Optimiser Approaches
- PCA Eigenportfolios as Statistical Risk Factors
The full lesson, with worked examples and exercise solutions, is for members.
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