The purpose of our research is to expand on the work of Kavvathas (2001) that studies credit rating transition probabilities for corporate bonds. This paper, for the period of 1991-2007 will be focused on rating transition matrices for US residential mortgage-backed securities (RMBS). In particular, we extend their techniques to a different data set and more recent time period by estimating credit rating transition matrices through the cohort method and the time-homogeneous duration method. In addition, we apply an alternative approach to calculate the average transition matrices
