This paper uses Monte Carlo simulations to assess the impact of noisy inputparameters on the accuracy of estimated portfolio credit risk. Assumptionsabout input quality are derived from the distribution of historical samplestatistics commonly used in default risk modelling. The resulting...

Rating agencies state that they take a rating action only when it is unlikely to be reversed shortly afterwards. Based on a formal representation of the rating process, I show that such a policy provides a good explanation for the empirical evidence: Rating changes occur relatively seldom,...