Having dropped to 5.1%, the unemployment rate has reached the longer-run employment goal of the Federal Reserve’s Open Market Committee (FOMC). So, starting to raise interest rates would seem to be in the cards. And, many observers expect policymakers to act soon, possibly very soon.

The key sticking point, and it is a big one, is that inflation – as measured by the personal consumption expenditure price index (PCE) favored by the FOMC – has been consistently below their stated 2% medium-term objective since early 2012.

Tightening monetary policy for the first time since 2006 requires confidence that inflation will in fact head back up (see, for example, the July FOMC statement and Fed Vice Chair Fischer’s recent comments). The difficulty is that confidence requires reliable forecasts. And, as it turns out, precise forecasts of inflation are hard to come by....