Specifically, empirical studies reveal that more than 99% of the movement of various Treasury bond yields are captured by three factors, which are often called “level,” “slope,” and “curvature” (Litterman and Scheinkman 1991). The names describe how the yield curve shifts or changes shape in response to a shock, as shown in Figure 1. Panel A of Figure 1 illustrates the influence of a shock to the “level” factor on the yield curve.