Structured finance ratings are assessments of the creditworthiness of ABS, CMBS, CLO, and other securitization bonds — but the analytical methodology differs substantially from how rating agencies evaluate corporate issuers. While corporate ratings assess the issuer's business model, management, and financials, structured finance ratings quantify the protection available to bondholders from the deal structure, the underlying collateral, and multiple layers of credit enhancement.
Rating agencies approach structured finance through a "bottom-up" model that begins with the underlying asset pool. For a pool of auto loans, analysts model the expected loss rate — the product of the default rate and the loss severity — under various scenarios. These expected losses are compared against the subordination, overcollateralization, and reserve accounts protecting each tranche to determine whether each tranche survives the required stress scenarios for its target rating.
Stress testing is the core analytical tool. For a AAA rating, the agency applies a "AAA stress scenario" — a severe recession scenario with dramatically elevated defaults and depressed recovery values — and determines whether the AAA tranche survives without principal loss. For AA ratings, a less severe but still stressed scenario is used. The required subordination level for each rating is the minimum amount of junior bonds needed to absorb losses under the corresponding stress scenario.
Structured finance ratings are not just set at closing — they are monitored throughout the life of the transaction. Monthly servicer reports provide pool performance data (delinquencies, defaults, prepayments, loss rates) that rating agencies review to determine whether ratings remain appropriate. Transactions experiencing significant collateral deterioration or structural stress may face negative watches, downgrades, or in severe cases, "impairment" of rated tranches.
The 2008 financial crisis exposed deep flaws in structured finance rating methodology — particularly in non-agency RMBS and structured credit — including reliance on historically benign housing price assumptions, flawed models of default correlation, and conflicts of interest in the issuer-pays model. Post-crisis rating agency reforms substantially increased stress levels and improved transparency, though the rating process for structured finance remains fundamentally dependent on the accuracy of the underlying quantitative models.