Evan Soltas has responded to me and others who don't quite see us as bumping up against full employment and capacity constraints any time soon. Fortunately, he clearly lays out his argument so it is easy to see the error in his ways. He notes the unemployment rate has fallen by 0.8 percentage points annually the last three years and assumes that we will continue on this path.

That one doesn't seem very likely unless Evan is either way more optimistic about growth than almost anyone else or way more pessimistic about productivity. The basic story is that unemployment has fallen more than most economists (including me) expected in the last three years. The reason is that productivity growth has slowed to a crawl. The average rate of productivity growth over this period has been less than 0.9 percent.

There is considerable debate about what sort of rate of productivity growth we should see going forward, but you won't find many estimates under 1.5 percent, and most would be around 2.0 percent. The average from 1995 to 2007 was 2.7 percent. If we use a 2.0 percent productivity number and assume 0.4 percent annual growth in the labor force (600,000 jobs a year), that gets 2.4 percent growth in potential GDP.

Most forecasts put GDP growth at the next three years around 3.0 percent. If we get productivity growth rebounding to its normal pace then we would be exceeding potential GDP growth by around 0.6 percentage points. Using historic relationships, this translates into a drop in the unemployment rate of roughly 0.3 percentage points a year. If our target is 5.0 percent unemployment (I'd shoot for lower, remembering the good old days of 2000 and 4.0 percent unemployment), we would get there in 2019.

This is why I and others are not anxious to see the Fed slam on the brakes any time soon. For what it's worth, any movement toward tightening any time soon would be seriously out of line with what the Fed did following the last downturn. It left the federal funds rate at 1.0 percent until the summer of 2004 when the unemployment rate was 5.6 percent and the economy was growing at almost a 4.0 percent annual rate. (Yes, they deserve to be strung up for allowing the housing bubble to grow to such dangerous levels, but that was a question of regulatory policy and targeting the bubble, not interest rates that were too low.)