The instruments of monetary policy have changed in significant ways as well. In the United States and some other countries, money and credit aggregates have played an especially important role as monetary policy instruments at times in the past.
However, the use of such variables as reliable guides for policy became increasingly problematic over recent years as increased competition and innovation in the financial sector tended to undermine the historical linkages between money and credit aggregates and key macroeconomic variables such as nominal GDP.