In this paper we show that this default premium is seriously flawed in two ways: (1) it is not based on subtracting maturity-matched government bonds from corporate bonds, and therefore contaminated with a considerable interest rate component, and (2) it is based on very high quality corporate bonds and hence rather insensitive to market-wide changes in default risk. These maturity and quality biases seriously limit the use of the Ibbotson default premium series in empirical research, because instead of reflecting pure default risk, it also (negatively) reflects interest rate risk.
Read The Journal of Investing, Vol.22, No. 2, 2013 (pp. 95-105)