A Risk-Factor Model Foundation for Ratings-Based Bank Capital Rules
Posted: 7 Dec 2004
I demonstrate that ratings-based capital rules, including both the current Basel Accord and its proposed revision, can be reconciled with the general class of credit value-at-risk models. Each exposure's contribution to VaR is portfolio-invariant only if (a) dependence across exposures is driven by a single systematic risk factor, and (b) no exposure accounts for more than an arbitrarily small share of total portfolio exposure. Analysis of rates of convergence to asymptotic VaR leads to a simple and accurate portfolio-level add-on charge for undiversified idiosyncratic risk. There is no similarly simple way to address violation of the single factor assumption.
JEL Classification: G31, G38
Suggested Citation: Suggested Citation