Based on the jobs data alone , the American economy is doing fabulously. Monthly payroll growth this year, averaging 267,000, already is ahead of last year’s impressive tally, which in turn handily beat the prior year. The unemployment rate, at 5.5%, is now in the range some economists consider “full employment.” But overall economic growth has been less impressive. That’s because productivity—the amount of goods and services each worker produces—is growing at a tepid rate. Since the economic expansion began in 2009, annual productivity growth has averaged just 1.3%, if the farm and government sectors are excluded. That is the weakest growth of any expansion since the 1970s. On Thursday, the day before the encouraging February jobs data were released, the Labor Department reported that productivity in last year’s fourth quarter didn’t grow at all […]