Risk.net Star quant proposes a new model for predicting changes in bond ratings Richard Martin speaking with Mauro Cesa Photo: Monika Ghose Risk.net’s Cutting Edge pages. Martin discusses his most recent paper, Credit migration: generating generators, in which he proposes improvements to classic Markovian models. Credit migration describes the evolution of upgrades and downgrades of bond ratings. It is commonly modelled using a matrix of parameters, known as generator matrix. But this approach has a known problem: a large number of parameters can lead to an unstable calibration, potentially making it difficult to identify the optimal solution.