In this paper,we summarize the establishment and development of structural models for measuring credit rating migration risks in recent years,especially in the case of assuming that the asset-debt ratio is used to divide the high and low ratings,where the different stochastic processes of different ratings are satisfied.This problem can be translated into a free boundary problem.The theoretical research results,computational and empirical methods of this model,as well as some generalizations and prospects of the model are presented.
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