Standard & Poor’s reported last week that of the 20,207 classes of mortgage-backed securities it has rated since 1978, only a fraction of them have defaulted since the first occurred in 1988. According to the report, only 0.93 percent, or 188, of the classes have defaulted, and Conseco Finance accounted for 20 percent of them. The rating service said 33 of the defaults reported in 2003 were directly related to various transactions originated by…