Statistical Surveillance of Structural Breaks in Credit Rating Dynamics

The 2007-2008 financial crisis had severe consequences on the global economy and an intriguing question related to the crisis is whether structural breaks in the credit market can be detected. To address this issue, we chose firms' credit rating transition dynamics as a proxy of the credit market and discuss how statistical process control tools can be used to surveil structural breaks in firms' rating transition dynamics. After reviewing some commonly used Markovian models for firms' rating transition dynamics, we present several surveillance rules for detecting changes in generators of firms' rating migration matrices, including the likelihood ratio rule, the generalized likelihood ratio rule, the extended Shiryaev's detection rule, and a Bayesian detection rule for piecewise homogeneous Markovian models. The effectiveness of these rules was analyzed on the basis of Monte Carlo simulations. We also provide a real example that used the surveillance rules to analyze and detect structural breaks in the monthly credit rating migration of U.S. firms from January 1986 to February 2017.