Blume, Marshall E., Felix Lim, and A. Craig Mackinlay, "The Declining Credit Quality of U.S. Corporate Debt: Myth or Reality?", Journal of Finance, Vol. 53, No. 4, (August 1998), pp. 1389-1413.
Abstract: In recent years, the number of downgrades in corporate bond ratings has exceeded the number of upgrades, leading some to conclude that the credit quality of U.S. corporate debt has declined. However, an alternative explanation of this apparent decline in credit quality is that the rating agencies are now using more stringent standards in assigning ratings. An ordered probit analysis of a panel of firms from 1978 through 1995 suggests that rating standards have indeed become more stringent, implying that at least part of the downward trend in ratings is the result of changing standards.