The information uncertainty created by the lack of disclosure on securitization transactions undertaken by banks led investors to rely even more on the ratings provided by rating agencies. In particular, most investors completely relied on agencies’ ratings of ABS senior tranches and did not develop their own models for assessing the underlying risk or carefully evaluate the assumptions made by rating agencies when assigning the ratings to ABS. As a consequence, investors did not understand that rating agencies were underestimating the risk of many ABSs, assigning AAA ratings to roughly 60 percent of all global structured products, in contrast to less than 1 percent of the corporate issues (Coval, Jurek,& Stafford,2008). As
Coval etal.(2008) point out, the credit ratings assigned by rating agencies depended on their ability to estimate with precision assets’ default risk and the correlation structure of the underlying assets. The estimation models rating agencies used were extremely sensitive to these assumptions, and even marginal errors could lead to dramatic changes in the rating assigned
to an ABS. To make matters worse, re-securitization practices amplified the effect of errors in the models’ assumptions because the effect of a change in the underlying assumptions at each securitization level was amplified. Coval etal.(2008) show that,for a mezzanine tranche of a CDO-squared, an increase of 2.5 percent in the default probability of the underlying assets could cause an AAA rating to drop to BBB. Since lending to subprime borrowers was arelatively recent phenomenon, historical data on defaults and delinquencies was scarce, making rating agencies’ tasks even harder and more prone to error. During the crisis, the probability of default and the expected recovery values were both worse than expected because of the deterioration in the credit quality of subprime borrowers and because assets were sold off under financial pressure in “fire sales” (Coval etal.,2008). As a consequence, most rating agencies’ assumptions proved to be wrong, along with their ABS ratings.