All investment assumptions are predicated upon market conditions, which in turn depends on government policy. Given that the government was pursuing a loose monetary policy, no one expected interest rates to rise, nor house values to decrease in the short term, looking at the figures alone. If the statisticians had done a field trip or an audit, things would have been much clearer. Moody's had succeeded in the past because they were able to quantify trust. It's failure today is precisely because they failed to audit some of these loans.