From the April 2009 FOMC minutes:

Some participants

highlighted the still-considerable uncertainty about the

future course of the financial crisis and the risk that a

resurgence of financial turmoil could adversely impact

the real economy. In addition, some noted the difficulty

in gauging the macroeconomic effects of the creditising

policies that are now being employed by the

Federal Reserve and other central banks, given limited

experience with such tools.