David Leonhardt at the times has a very good article on unemployment statistics, and it explains how it has become less accurate over time.

Consider this: the average unemployment rate in this decade, just above 5 percent, has been lower than in any decade since the 1960s. Yet the percentage of prime-age men (those 25 to 54 years old) who are not working has been higher than in any decade since World War II. In January, almost 13 percent of prime-age men did not hold a job, up from 11 percent in 1998, 11 percent in 1988, 9 percent in 1978 and just 6 percent in 1968.

It’s a good read, and better written than I can do.