Please don´t miss this wonderfully readable discussion of reasons for the adoption of an NGDP Level Target by Nick Rowe.
Nominal GDP is just one of an infinite number of variables that monetary policy might target. The probability that NGDP is exactly the best target variable is infinitesimal. I nevertheless support NGDP targeting, because I think it is probably reasonably close to that unknown best target variable; and I think NGDP level path targeting is probably better than reasonable alternatives like inflation or price level targeting.
Historinhas 