What is reverse stress testing and how do I actually run one?

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AcadiFi · Accepted Answer

Reverse stress testing inverts the standard question. Instead of 'given this scenario, what is my loss?', it asks 'what scenarios would cause my business to fail or breach a specific threshold?' The threshold is typically capital exhaustion, liquidity insolvency, or loss of a critical franchise.

Operationally, there are three approaches. First, analytical search: use a factor model to identify factor combinations that produce a target loss using optimization (e.g., most-likely scenario on a surface of equal loss) subject to a plausibility constraint like Mahalanobis distance under historical covariance. Second, qualitative expert workshops: assemble business heads and risk to brainstorm scenarios that would break the firm — loss of a major counterparty, run on wholesale funding, fat-tail model failure — and then translate each into a quantitative scenario. Third, Monte Carlo search: simulate many scenarios and filter those producing the target loss, then cluster them into narrative themes.

For a mid-size broker-dealer like Quartzmill Securities, capital exhaustion might correspond to a $1.4B loss against $1.6B of CET1. A reverse stress test might surface: a 40% equity crash combined with a 300bp credit widening and loss of prime brokerage financing — a 1-in-250-year scenario but one the firm must plan recovery actions for.

Regulators (PRA, FRB CCAR, ECB SSM) require reverse stress tests and explicitly want the firm to identify binding constraints and pre-approved recovery actions. FRM exam point: reverse stress tests are the primary input to recovery and resolution planning.

What is reverse stress testing and how do I actually run one?

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