Perceived credit quality and risk exposure
For purposes of risk management bonds are often grouped according to agency ratings based on the assumption that bonds with similar ratings tend to show a high degree of comovement. Breger et al. (2003) examine whether the correlation between individual bonds increases if they are grouped by implied ratings, that is by spread classes rather than by agency ratings. The rationale for this would be that market valuations are a better indicator for the drivers of credit spread changes, namely perceived credit quality and risk exposure, than are agency ratings. In their empirical study they find that bonds of the same spread class are more similar than bonds with the same rating from a risk/return perspective. Breger et al. (2003) conclude that the classification of bonds based on market data provides a more reliable basis for modeling return relationships than does a classification by agency ratings. However, one has to note that the motivation behind this study differs significantly from the rating agencies’ approach. The objective is not to predict default risk, but rather to improve the classification of corporate borrowers and provide a basis for reliable spread risk forecasts.