There is ongoing pressure from regulators, investors and rating agencies on financial institutions to build appropriate models that allow measurement of the different risks faced. For the financial institutions the implementation of these models is often a first step towards developing what is now often called an enterprise wide risk framework, which can support and reward management on an enterprise-wide basis by integrating all risk components. As far as credit risk is concerned it follows from Sklar’s theorem that one is able to assess the risk of the entire loan portfolio provided that the dependency structure or copula is known (as well as the marginal distributions of the individual credit losses). However, whilst the assessment of the marginal risks is now relatively well under control, fitting a copula in a credit context is a difficult exercise due to the relative scarcity of observed defaults. Therefore, most institutions limit themselves to using default correlations when assessing the dependencies and use a variance-covariance framework to asses the credit portfolio risk. It must be noted that in the literature on default correlations it is now standard to use the concept of asset correlation to discuss and to compare the different findings. Indeed, an assumption on joint asset movements (typically using a Gaussian copula) allows one to back out the implied asset correlations from the default correlations; we refer to Crouhy et al (2000) for more details.
Asset correlations are also an important component of the Basel II Accord for regulatory capital requirements of credit risk portfolios. In the Basel Committee on Banking Supervision (BCBS) document of January 2001 (BCBS (2001a), asset correlations were assumed to take a value of 20% for all obligors. The modification later that year (BCBS (2001b)) assumed that asset correlation declined with PD: for the lowest PD the asset correlation was 20% and for the highest PD the asset correlation was 10%. Then finally in the Consultative Document of 2006 asset correlations for sovereigns, banks and corporates were principally assumed to be between12% and 24%, once again depending on the probability of default. We note that for small and medium sized corporates an extra downward firm-size adjustment up to 4% is made and this brings the effective range of corporate asset correlations between 8% and 24%.