With this foundation, the literature began to apply credit risk in new arenas and relax some key assumptions. Hull and White (1995) discuss how one can incorporate counterparty credit risk in derivative securities, while Duffie and Singleton (1999), and Jarrow and Turnbull (1997) focus on the dynamic nature of credit risk through credit spreads. Duffie and Singleton prove the uniqueness of credit spreads and apply them to a variety of contexts, including term structure models and derivative products; Jarrow and Turnbull develop a model that allows for dynamic credit spreads and calculate the associated impacts on pricing. Lando (1998) demonstrates how risk-free interest rates can be incorporated into quantitative credit analysis. This generalization dramatically increased the potential for quantitative default modeling: now, one can reasonably look at a variety of inputs or “state variables” when characterizing credit risk rather than asset value, volatility and leverage as in Merton (1974).