The reason for this high rating was the there was a diversification effect affect the whole pool significantly. Using historical patterns and simulation analysis, credit rating agencies consistently found ways to rate mortgage backed securities as AAA even though they paid more interest than other AAA obligations. The high yields on AAA rated mortgage backed securities attracted investors and created a demand for more mortgage-backed products. Through what is called “financial engineering,” derivative mortgage instruments called collateralized debt obligations (CDOs) were created that invested not in mortgages, but in mortgage-backed securities and pool them together in a CDO that the credit rating agency would bless with an AAA rating.