Specifically, it distinguishes between endogenous and exogenous segmentation risk drivers and, through application to a portfolio of first mortgages, shows that risk weights remain stable over the economic cycle when the segmentation scheme is derived using exogenous risk drivers, while segmentation schemes that include endogenous risk drivers are highly pro-cyclical. Also analyzed is the sensitivity of the A-IRB framework to model risk resulting from the selection, at the quantification stage, of a data sample period that does not include a period of significant economic downturn. The analysis illustrates important limitations and sensitivities of the A-IRB framework and sheds light on the implicit restrictions embedded in recent regulatory guidance that underscore the importance of rating systems that remain stable over time and throughout business cycles.