Duffie, Wang, and Saita (2007) illustrate how to incorporate default risk that varies over time through a conditional term structure of default probabilities based on observable characteristics. This naturally leads to questions concerning correlation in default intensities over time. Das, Duffie, Kapadia, and Saita (2007) and Jarrow and van Deventer (2005) examine endogenous correlation in a reduced form model through dependence on common covariates.